[Nakamoto] signers fall behind if they fail to sign a block by a miner's timeout

[jferrant]

Currently signers will sign blocks in the order that they arrive. However, if a miner proposes a block while they are mid signing another, it means they have given up on that block and are expecting a new signature to arrive. however, the signers must currently finish that round before they handle the new block proposal which ends up making them fail to sign it in time for the miner again, falling every more behind/ensuring no block ever gets mined. Have the signers either immediately abort a round if a new block arrives or alternatively have the miner accept the first block signature they receive back for one of their prior proposed blocks. (means these blocks must be tracked by the miner though...)