Provide hooks to integrators for alerting after a step is stuck for too long

[masih]

GPBFT rebroadcasts a set of messages when stuck at a step with increasing rebroadcast timeout. But it is not strictly specified what the protocol should do when stuck indefinitely.

In practice, human intervention is most likely necessary to unstuck the step by e.g. feeding finality certificates, etc. Systems like lotus already have an alerting mechanism for such cases.

Ideally, the F3 implementation should offer a way for integrators to receive alerts when stuck, while F3 would internally rebroadcast messages indefinitely with an exponentially increasing timeout capped at some max threshold.