Test HF activation accross reorg

Summary: This tests that after reorg the replay protected transaction limited to the HF are rejected from the mempool. Right now, we just clear the mempool. This is somewhat of a blunt instrument but it will do the job.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py abc-p2p-activation.py

Reviewers: freetrader, sickpig, awemany, kyuupichan, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D213