Trigger UAHF based on height

Summary: This simplify various things and a 2 month reorg is not on the table.

Test Plan:

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

Reviewers: freetrader, schancel, dagurval, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D558