[signer] Do not submit more than one block validation request at a time

[jferrant]

Sometimes nodes get so far behind that it doesn't even return 429 to multiple block proposal submissions. This results in signers bombarding the node with proposals. It would be better to track if the signer has submitted a proposal, and not submit another until a timeout is reached or a response returned. If a timeout is reached, the proposal should be marked as rejected regardless of what the node later responds back. Make this timeout a configuration option.