p2sh txns added to the mempool

Summary: p2sh txns were added to the mempool, for the task https://reviews.bitcoinabc.org/T47

Test Plan: python3 rpc-tests.py mempool_p2sh_txn.py

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: freetrader, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D239