Add activation code for OP_CHECKDATASIG

Summary:
As per title.

Depends on D1623 and D1621

Test Plan: Added an integration test to check for proper activation and deactivation.

Reviewers: #bitcoin_abc, jasonbcox, schancel

Reviewed By: #bitcoin_abc, jasonbcox, schancel

Subscribers: jasonbcox, schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1625