Refactor flag selection in AcceptToMemoryPoolWorker

Summary: This'll make it simpler to activate/deactive several flags.

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1230