Make push only mandatory when magnetic anomaly activates.

Summary:
As per title.

Depends on D1622 and D1611

Test Plan: Added test cases in abc-magnetic-anomaly-activation

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1623