Add activation code for the block size parameter of UAHF.

Summary:
This ensure no block larger than 1MB is mined before the fork. After the fork, larger blocks can be mined.

Depends on D190

Test Plan:

make check
../qa/pull-tester/rpc-tests.py

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

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D194