Garbage collect "outdated" outgoing protocol messages

[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:
...

• 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 Outgoing messages buffered in the I/O abstraction can be canceled/withdrawn, replaced with CHECKPOINT messages for the current round.