Implement speedybadger's state file persistence

[sbellem]

draft

speedybadger is meant to be a kind of SPV-like (bitcoin) or fast-sync (ethereum) mechanism.

This issue is concerned with the "state file", as mentioned in amiller/HoneyBadgerBFT#57:

Observation 2. Although the entire blockchain of committed transactions (T is the total number of transactions) grows unboundedly, the size of the current “state” S at any given time is typically much smaller and may be considered bounded.

For example, |S| may be bounded by the number of active accounts, whereas |T| is the number of transactions made by all the accounts.

State S_1, ,... S_B where S_B = apply(S_{B-1}, B.txs).

Hence if a node falls many blocks behind (or even if it crashes and restarts), then it can “catch up” to the current block by downloading just the current state S rather than the entire log of transactions. This is known as SPV-syncing in Bitcoin, and fast-sync in Ethereum.