Akin to #268, but the prerequisite bitcoin/bitcoin#35671 is expected to land much later than bitcoin/bitcoin#34020.

The TxCollection offers two opportunities:

1. Get rid of mempool.rs and monitors.rs (or keep them only for backward compatibility)
2. Hold on to the reconstructed block template and use the (slightly) more efficient submitSolution() instead of submitBlock()

Demo:

• PoC: replace the mempool mirror with Bitcoin Core's TxCollection #599