Check for equivocations across messages received and justification

[masih]

Messages with justification contain evidence of prior messages (either COMMIT or PREPARE) from current or previous round. Currently it is possible for nodes to equivocate across them, e.g. send PREPARE for one value and change it in the justification of consecutive COMMIT.

Fix:

• Treat reception of justifications as if reception of the messages they evidence #342

[masih]

Update: the fix for this is not exactly getting #342 done. It is actually a smaller subset of related changes. I will approach fixing this specific issue first before working on #342. Because, #342 would require revise of how justifications vs. individual message signatures are stored in the quorum state.

[masih]

Discussed in standup: Although it is possible for a node to equivocate (and such messages are accepted by gpbft package), we decided to not fix this issue. Because:

1. for this to be an effective attack vector the adversary must have at least 1/3 of the power, at which point there are other things to worry about, and
2. we cannot censor equivocating justifications or the drop the original in favour of the one in justification because that would open up a new censorship attack vector where an adversary with enough power can cause a network partition.