[Chronik] Move mempool module to chronik-db

Summary:
The mempool could be seen as a kind-of in-memory DB for unconfirmed txs, so this certainly isn't out of place.

Having the mempool here also allows for better synergies, e.g. reusing functions specific to Groups.

The legacy Chronik implementation also has the mempool in the chronik-db crate, somehow this was overlooked when porting.

Depends on D13487.

Test Plan: ninja check-crates

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13500