Treat reception of justifications as if reception of the messages they evidence

[anorth]

A vote justification carries evidence of a strong quorum of nodes all having sent the same message. If a participant receives a justification but has not received those messages, it can treat the justification as those messages. For example, if a node is stuck in PREPARE but receives a COMMIT message, it should be able to use the justification from that COMMIT to immediately complete the PREPARE phase.

[rjan90]

@Stebalien / @Kubuxu Is this still open for discussion, or should we do this?

[Stebalien]

We should but it's probably not the highest priority.