spec/keeper: Adds Skip threshold and threshold emission tracking

[angbrav]

The vote bookkeeper now emits a skip threshold when f+1 for any value in a given round is reached. This PR also adds logic to avoid emitting the same threshold multiple times (only emitted the first time the quorum is reached).

Emission logic:

• After applying a vote, the keeper always returns an event.
• Each event is emitted once, except for "None" which can be emitted an unbounded number of times.
• The keeper emits a Skip event only if no quorum event has been previously emitted for that round.

[josef-widder]

looks good. thanks

[romac]

Looks good, and was very helpful when working on #39