Current code allows Commit to contain precommits for the wrong blockID to facilitate tracking validator liveness.

Opened #3484 to actually test for this.

In working towards removing redundancy in commits (#1648), it was realized that we need to track which blockID a precommit was for if they're not all for the same blockID. For the +2/3 that are for the right blockID, this is redundant information. But for the others, it's not.

This raises the question of if we really even need to include these precommits in a commit in the first place.

I think we certainly don't need to include those that are for a different blockID. So maybe the question is more so if we need to include precommits for nil. Even in this case, I think not, especially since to do signature aggregation we need all the msgs to be the same, unless we want to aggregate two sets of votes independently - those for the blockID and those for nil. Not sure this is worth it.

@zmanian @jaekwon @milosevic @cwgoes @sunnya97 thoughts? Can we just require Commit only contains precommits for a single blockID?