Handle validator set changes in the hbbft engine.

[afck]

Whenever a validator set change is detected, a new distributed key generation session needs to be started:

All future validators must instantiate a SyncKeyGen and send and handle all messages using the KeyGenHistory contract. Once key generation is complete, the HoneyBadger instance needs to be replaced with a new one, using the new NetworkInfo, and starting at the next block number.