beacon: persist beacon protocol state and recover on startup

[dshulyak]

i think there are two important events when we should persist:

• after proposals and first round votes were received
• on shutdown

we should also drop persisted data, when

• node starts
• epoch completes (or next starts)

[countvonzero]

as per research discussion on Aug 28. because windows doesn't currently do graceful shutdown #4909

the proposal is modified to persist state periodically (every round)

[mathcrypto]

So the goal is to drop all persisted data at end of epoch N? How would that impact epoch N+1 to retrieve beacon from epoch N?