Move the "container updated" message for Buildables that build Diffs outside of the transaction

Summary:
Ref T13216. See PHI970. Ref T13054. See some discussion in T13216.

When a Harbormaster Buildable object is first created for a Diff, it has no containerPHID since the revision has not yet been created.

We later (after creating a revision) send the Buildable a message telling it that we've added a container and it should re-link the container object.

Currently, we send this message in applyExternalEffects(), which runs inside the Differential transaction. If Harbormaster races quickly enough, it can read the Diff object before the transaction commits, and not see the container update.

Add a didCommitTransaction() callback after the transactions commit, then move the message code there instead.

Test Plan:

• See T13216 for substantial evidence that this change is on the right track.
• Before change: added sleep(15), reproduced the issue reliably.
• After change: unable to reproduce issue even with sleep(15) (the containerPHID always populates correctly).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13216, T13054

Differential Revision: https://secure.phabricator.com/D19807