Check transaction that enter the mempool against the flags required for the current block rather than a minimal set of mandatory flags.
Summary:
This is based on work by Matt Coralo. We shouldn't accept in the mempool any transaction that won't be valid if mined.
Test Plan:
make check ../qa/pull-tester/rpc-tests.py -extended
Reviewers: freetrader, CCulianu, #bitcoin_abc, dagurval
Reviewed By: #bitcoin_abc, dagurval
Differential Revision: https://reviews.bitcoinabc.org/D527