Use disconnectpool in CTxMemPool::removeForBlock

Summary: This ensure that transaction are removed in a consistent order. It's preferable to do it that way untill the mempool code is able to take advantage of it by itself.

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: hanchon, schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1686