More compact encoding for Commit

Problem with commit is it's quite large _and_ have to stored forever. So it's desirable to store it in most compact form possible. It's in fact quite redundant: most (2/3 at least) vote for the same BlockID but currently they're stored independently. Possible improvements:

1. Store commit as `Map (Maybe BlockID) (Address, Signature)` thus avoiding duplication of lock hash

2. We know all possible key in commit so we can go one step further and store them as `Map (Maybe BlockID) (Int,Signature)`, where Int is number of public key in sorted list.