Implement a flag to trigger replay protection when passed down to SignatureHash

Summary: This will allow to enable replay protection at any point in time.

Test Plan:
Extend sighash_test to test for sighash generation with various sets of flags instead of just one.

Depends on D1190, D1191

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1192