In #3298 we seek to explicitly exercise code paths necessary to reconstruct Vote from CommitSig even before we reduce the fields in CommitSig (ala #1648).

If CommitSig does not include the validator address directly, this requires loading the current validator set, which leads to a question of how to handle an error if there is one, which there shouldn't be - see https://github.com/tendermint/tendermint/pull/3298/files#diff-f50d554d9c7f92cf7686b247b8c91419R642.

Alternatively, we may need to keep the addresses in the CommitSig to support light clients properly, which should eliminate the need to load validator sets, but it means 2k less in savings in block space for 100 validators (ie. 20byte addresses).