Make it possible to coalesce distinct pubsub messages

[Stebalien]

If we split pubsub message IDs into "validation IDs" and "propagation IDs", we'll be able to drop equivalent pubsub messages even if they're not 100% identical.

• At the validation layer, use the message hash to deduplicate (make sure to make this work with rebroadcast).
• After successful validation, identify the message based on the hash of the gpbft value.
• Only propagate messages with unique GPBFT values.

[Stebalien]

IIRC, there were two motivations here:

1. An improved finality certificate protocol. But that can always be punted till later.
2. Avoiding justification validation. Given Change BLS signature/public key aggregation to enable better caching #592, I'm not sure if that's still critical. Validation of justification signatures is just as fast as validating the message so, at most, we'd cut validation time by 1/2 (but I'm not sure if that's worth it, also Batch validate justification + message signature #614).

[Stebalien]

We're deferring this because it's not a huge priority, see above.