Maintain mempool caches across Stacks blocks

[obycode]

During mempool iteration, the miner users two caches

• a nonce cache, to track the current nonces of accounts
• a candidate transaction cache, to save transactions that may soon be eligible to include in a block

These caches currently do not persist in between blocks since in the original pre-Nakamoto design, each miner only mined one Stacks block. We might get some significant benefit from maintaining these caches across blocks.