The mempool could be seen as a kind-of in-memory DB for unconfirmed txs, so this certainly isn't out of place.
Havint the mempool here also allows for better synergies, e.g. reusing functions specific to `Group`s.
The legacy Chronik implementation also has the mempool in the `chronik-db` crate, somehow this was overlooked when porting.
Depends on D13487.