More compact encoding for Commit

[Shimuuar]

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.

[Shimuuar]

We also want to generalize to BLS aggregate signatures. Possible approach which will work for both DBL & Ed25519 schemes is to treat commit as threshold signature for the vote. It necessarily requires throwing out time from Vote since all votes in this case must be equal.

So we need to implement aggregate signatures for both BLS & Ed25519. Latter has support only for trivial multisignatures (list of signatutres)

• Public membership. It's known which keys participated in signature
• Weighted keys / more general predicate on signing set validity.