Add helper function to derive the reward cycle from NakamotoBlock to which the block belongs

[jferrant]

Signers trust that the reward cycle listed in a BlockProposal is the valid reward cycle of the block being proposed. It will cause them to store it in their signerdb against this reward cycle. I do not think we should explicitly trust this value as a malicious miner could bypass reward cycle checks in the signer by simply overwriting this value. The wrong signers could inadvertently sign a block proposal and then enter into an infinite loop attempting to broadcast it to the node when its actually invalid.

Signers should use this helper function when storing the Block inside their signerdb instead of the one listed in the block proposal.