Code review comment for lp:~niemeyer/juju-core/presence-polishing

Reviewers: mp+123627_code.launchpad.net,

Message:
Please take a look.

Description:
mstate/presence: bring it in line with mstate/watcher

This mirrors into mstate/presence the improvements made into
the mstate/watcher package, including several points made in
reviews.

It also addresses a bug in Alive (it could return false
improperly without errors).

https://code.launchpad.net/~niemeyer/juju-core/presence-polishing/+merge/123627

Requires:
https://code.launchpad.net/~niemeyer/juju-core/mstate-machine-watcher/+merge/123614

(do not edit description out of merge proposal)

Please review this at https://codereview.appspot.com/6501114/

Affected files:
   A [revision details]
   M mstate/machine.go
   M mstate/machine_test.go
   M mstate/open.go
   M mstate/presence/presence.go
   M mstate/presence/presence_test.go
   M mstate/state.go
   M mstate/unit.go
   M mstate/unit_test.go
   M mstate/watcher/watcher_test.go