Finalize each block with a threshold signature

[sbellem]

From amiller/HoneyBadgerBFT#57:

After finalizing each HBBFT-Block, nodes produce a t+1 threshold signature on a merkle tree over all the transactions, as well as a merkle tree over the current state file S. This signature serves as a CHECKPOINT, a succinct piece of evidence that the current block has concluded. CHECKPOINTs are used for three different purposes:

• I. Prevent DoS from malicious nodes. Messages for future rounds B' > B are ignored until a CHECKPOINT message for round B is received. This enables a node to discard DoS messages sent from an attacker, which would otherwise appear to be plausible messages in the future.

• II. Allows lagging (or restarted) nodes to recover. When a node receives a valid CHECKPOINT for a later block than the current one (for block B’ > B), then it determines it has fallen behind. It deallocates any buffered incoming or outgoing messages in block B, and

• III. Allow nodes to garbage collect old outgoing messages. Because nodes that have fallen behind can catch up via a CHECKPOINT, it is not necessary to buffer outgoing protocol messages pertaining to earlier blocks. Outgoing messages buffered in the I/O abstraction can be canceled/withdrawn, replaced with CHECKPOINT messages for the current round.