Cherry-pick bot should monitor changes it stages

On somewhat rare instances, a change will be staged in gerrit and never picked up by COIN. When a user stages a change that then hangs, they usually act to fix the issue by unstaging and restaging the change. If Cherry-pick bot is the only user added to a change that hangs, no one is watching and the change may get left behind.

 To work around COIN's unreliability in this, Cherry-pick bot should watch changes it stages and make sure that the status moves to INTEGRATING after a period.

Proposal:

Poll for INTEGRATING after 24 hours. Allow incoming messages about "change-merged", "change-integration-fail" to cancel the polling. If the status remains STAGED after 24 hours, try to restage the change.

Further, monitor for change-integration-failed events to take action by either restaging or alerting a user.