Make the UAHF start time configurable

Summary: As per title. This was a requirement.

Test Plan:

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

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

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D221