pool: Backfill from event listener for persistent mempools.

[dancoombs]

Describe the feature
For persistent mempools (DB/Redis backed) the node will need to backfill from the event listener to remove any UOs from the persistent pool that have already been mined. This should be a simple extension to the event listener to provide a pull method from a range of blocks and return an aggregated set of block events (with UO events).

Additional context
This doesn't apply to ephemeral mempools, like the P2P mempool or the closed-beta mempool.

[dancoombs]

Closed by #276