Parse UAHF start time as integer constant
Summary:
There is no need to eval the start time -
just parse it as an integer.
Test Plan: ../qa/pull-tester/rpc-tests.py
Reviewers: #bitcoin_abc, deadalnix, sickpig, awemany, CCulianu
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D317