Enforce that a large block is produced when the HF triggers

Summary: This is as per spec. The first block of the HF must be larger than 1MB in order to avoid reorg.

Test Plan:
  make check

Added integration tests to check for the proper activation of the fork.

Reviewers: freetrader, sickpig, awemany, kyuupichan, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D200