Consolidate signer <-> stacks-node communication

[obycode]

When a signer receives a block proposal, there are many RPC calls to the stacks-node. Each call is very quick to process, but sometimes we see that it takes a while for the stacks-node to start processing each call. This adds up to a bunch of unnecessary wait time in the signer. In practice, we've seen it take 15 seconds between the time the signer receives the block proposal and the time it sends the block validation request.