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

We should add tests for this, both for the case of nil-votes and of votes for the wrong blockID. Both are allowed as valid votes in a Commit, so long as the Commit contains +2/3 votes for the right block ID.

Note this was discovered in #3298 which aimed to be the last step in implementing #1648 (ie. removing redundancy from commit votes).

Ultimately, we maybe don't want to allow votes for the wrong blockID, but just for nil.