Support for nodes dropping out during the protocol

[fjarri]

Rounds will need to provide a "strategy" describing what nodes they're expecting messages from, and which messages are enough to finalize. Probably returned from Round::expecting_messages_from instead of the current BTreeSet. Possible options include: all of the nodes, a threshold, maybe several separate sets with their own threshold.

Echo round will need to be mindful of these strategies too when implementing its own expecting_messages_from and checking for errors.