[nakamoto] signers should process their pending blocks before ending their tenure

[jferrant]

Suppose we are in the last block of cycle N, the signers receive a block proposal and they submit it for validation: while waiting for a response, a new burn block arrives starting cycle N+1, and terminating signers tenure. Currently, the signers do not continue processing the pending block and simply terminate their tenure. However, the signers SHOULD process the pending block they have already submitted for validation before they terminate their tenure. Make this change and add a test.

[blockstack-devops]

This issue has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.