Anti replay OP_RETURN feature.

Summary: When a user makes a transaction with a certain value as OP_RETURN, this transaction needs to be ignored on the HF chain. Values for activation and comitment value are placeholder and are yet to be communicated.

Test Plan: Added unit tests.

Reviewers: sickpig, freetrader, kyuupichan, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D148