COMMIT messages and skipping ahead rounds

[rphmeier]

A safe protocol for skipping ahead in rounds in a voter would be useful. Perhaps it can be done outside, but with a utility provided by this crate.

COMMIT messages need to be fleshed out for observers to more easily witness finality.

[rphmeier]

About skipping ahead in rounds:

We need logic in VotingRound for not voting when last_round_estimate is None.

The voter keeps an active_round: VotingRound and a prospective_skip: Option<VotingRound>;

Whenever we see a commit for round r > active_round.round + 1, we start prospective_skip at r+1 with no last_round_estimate.

Then one of three things will happen:

• We see an even further commit and overwrite prospective_skip
• active_round works its way up to r and becomes completable. In this case, we background active_round, set active_round = prospective_skip.take() and set the new active_round.last_round_estimate accordingly.
• prospective_skip becomes completable, and we background it and start active_round at r+2.