Snap build request changes cause snap builds to fail with DB permission errors

Following r18956, dispatching snap builds fails as follows:

2019-05-07 12:47:07+0000 [-] Traceback (most recent call last):
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 500, in errback
2019-05-07 12:47:07+0000 [-] self._startRunCallbacks(fail)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 567, in _startRunCallbacks
2019-05-07 12:47:07+0000 [-] self._runCallbacks()
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 653, in _runCallbacks
2019-05-07 12:47:07+0000 [-] current.result = callback(current.result, *args, **kw)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1442, in gotResult
2019-05-07 12:47:07+0000 [-] _inlineCallbacks(r, g, deferred)
2019-05-07 12:47:07+0000 [-] --- <exception caught here> ---
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1384, in _inlineCallbacks
2019-05-07 12:47:07+0000 [-] result = result.throwExceptionIntoGenerator(g)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/twisted/python/failure.py", line 422, in throwExceptionIntoGenerator
2019-05-07 12:47:07+0000 [-] return g.throw(self.type, self.value, self.tb)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/lib/lp/buildmaster/interactor.py", line 515, in findAndStartJob
2019-05-07 12:47:07+0000 [-] candidate, vitals, builder, slave, new_behaviour, logger)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1384, in _inlineCallbacks
2019-05-07 12:47:07+0000 [-] result = result.throwExceptionIntoGenerator(g)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/twisted/python/failure.py", line 422, in throwExceptionIntoGenerator
2019-05-07 12:47:07+0000 [-] return g.throw(self.type, self.value, self.tb)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/lib/lp/buildmaster/interactor.py", line 488, in _startBuild
2019-05-07 12:47:07+0000 [-] yield behaviour.dispatchBuildToSlave(logger)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1384, in _inlineCallbacks
2019-05-07 12:47:07+0000 [-] result = result.throwExceptionIntoGenerator(g)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/twisted/python/failure.py", line 422, in throwExceptionIntoGenerator
2019-05-07 12:47:07+0000 [-] return g.throw(self.type, self.value, self.tb)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/lib/lp/buildmaster/model/buildfarmjobbehaviour.py", line 116, in dispatchBuildToSlave
2019-05-07 12:47:07+0000 [-] self.composeBuildRequest(logger))
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1384, in _inlineCallbacks
2019-05-07 12:47:07+0000 [-] result = result.throwExceptionIntoGenerator(g)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/twisted/python/failure.py", line 422, in throwExceptionIntoGenerator
2019-05-07 12:47:07+0000 [-] return g.throw(self.type, self.value, self.tb)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/lib/lp/buildmaster/model/buildfarmjobbehaviour.py", line 98, in composeBuildRequest
2019-05-07 12:47:07+0000 [-] args = yield self.extraBuildArgs(logger=logger)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1386, in _inlineCallbacks
2019-05-07 12:47:07+0000 [-] result = g.send(result)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/lib/lp/snappy/model/snapbuildbehaviour.py", line 161, in extraBuildArgs
2019-05-07 12:47:07+0000 [-] timestamp = format_as_rfc3339(build_request.date_requested)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/lib/lp/snappy/model/snap.py", line 213, in date_requested
2019-05-07 12:47:07+0000 [-] return self._job.date_created
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/lib/lp/services/propertycache.py", line 115, in __get__
2019-05-07 12:47:07+0000 [-] value = self.populate(instance)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/lib/lp/snappy/model/snap.py", line 208, in _job
2019-05-07 12:47:07+0000 [-] return job_source.getBySnapAndID(self.snap, self.id)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/lib/lp/snappy/model/snapjob.py", line 231, in getBySnapAndID
2019-05-07 12:47:07+0000 [-] SnapJob.job_type == cls.class_job_type).one()
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/storm/store.py", line 1179, in one
2019-05-07 12:47:07+0000 [-] result = self._store._connection.execute(select)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/storm/databases/postgres.py", line 306, in execute
2019-05-07 12:47:07+0000 [-] return Connection.execute(self, statement, params, noresult)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/storm/database.py", line 241, in execute
2019-05-07 12:47:07+0000 [-] raw_cursor = self.raw_execute(statement, params)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/storm/databases/postgres.py", line 316, in raw_execute
2019-05-07 12:47:07+0000 [-] return Connection.raw_execute(self, statement, params)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/storm/database.py", line 374, in raw_execute
2019-05-07 12:47:07+0000 [-] self._run_execution(raw_cursor, args, params, statement)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/storm/database.py", line 388, in _run_execution
2019-05-07 12:47:07+0000 [-] self._check_disconnect(raw_cursor.execute, *args)
2019-05-07 12:47:07+0000 [-] File "/srv/launchpad.net/codelines/soyuz-production-rev-18958/env/local/lib/python2.7/site-packages/storm/database.py", line 454, in _check_disconnect
2019-05-07 12:47:07+0000 [-] return function(*args, **kwargs)
2019-05-07 12:47:07+0000 [-] psycopg2.ProgrammingError: permission denied for relation snapjob