Compute max generated block size depending on UAHF activation

Summary: We don't want to allow block more than 1MB to be generated before UAHF activation.

Test Plan: Added tests that check the generated block size before and after activation.

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

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D206