Call TransactionRemovedFromMempool in the CScheduler thread

Summary:
This is both good practice (we want to move all such callbacks
into a background thread eventually) and prevents a lock inversion
when we go to use this in wallet (mempool.cs->cs_wallet and
cs_wallet->mempool.cs would otherwise both be used).

This is extracted from Core's PR10286 commit 3ea8b75281edc60078423bd5d277cd2a84aa5d33

Depends on D2263

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2265