GerritStatusPush: missing retrying mechanism for staus updates

[PiotrSikora]

Any status updates sent to Gerrit that won't be accepted (either because they were rejected or because connection failed) are currently lost.

Buildbot should queue and retry sending them later (ideally it should store them in persistent storage, so they would survive restarts, but that's probably asking for too much...).

I think that same might be true for IRC, e-mail and other status updates.