Should we *only* vote on blocks which are in the best branch of fork_db?

[greg7mdp]

Right now, we allow a vote only on blocks whose latest_qc_claim block or one of its descendants has been validated.

Since we can only be sure that the blocks in the best branch are validated, this implies that we also want to only vote on blocks that are in the best branch (otherwise the vote may fail unpredictably if we attempt to vote on a block not in the best branch, depending on whether or not that branch was ever the best branch and its blocks were validated).

Also, should we vote on a block whose core.latest_qc_claim() is for a block that is not in the best branch of fork_db? Right now we will vote iff that branch was the best branch at least once in the past (so the core.latest_qc_claim() block was validated).