Consensus extra commit logic to avoid waiting for 100% sigs

[rlan35]

This PR implement the extra commit signature logic which allow leaders to only wait for 2/3 of the commit signature in current block and include the rest of the signatures (extra commit sig) in next block. Specifically, following changes will take effect after the extra commit fork epoch:

1. ExtraCommitSig and ExtraCommitBitmap field is added to block header v4 which only take effect after ExtraCommitEpoch.
2. Leaders will immediately finalizing the block once it received more than 2/3 of the voting powers on commit sig.
3. In the next block, the leader will accept the rest of the commit signatures from the previous block and store it as extra commit signatures in DB and include it in the proposal in the new block.
4. Validators is allowed to sign commit signature on the previous block so the signature can be included as a extra commit sig even if the validator is slow and didn't make it in the first 2/3 of the signatures.
5. After the fork epoch, crosslinks and block rewards are calculated by combing the extra commit sig and the normal 2/3 of the sig together. The block reward distribution logic stay the same.

Fully tested this change on local network and was able to see extra commit signature being signed and accepted, crosslinks constructed/accepted and block rewards distributed correctly.

[MaxMustermann2]

Does the memory impact of this additional cache need to be assessed ?

[rlan35]

It's a 10 slot cache, which will be close to nothing to store 10 signatures. So not a concern. But good to check.