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