Code review comment for lp:~wgrant/launchpad/bug-507784

BuildFarmJobs with builds have some common behaviours. This branch moves them (jobStarted, jobReset and jobAborted) into a new common base class, PackageBuildFarmJob. The name sucks, but neither I nor jml could think of anything much better.

I've also set a sane default build log filename, 'buildlog.txt'. Binary package builds will still use their insanely long custom variant.