Implement setting of UAHF start time via RPC

Summary:
Implement getuahfstarttime / setuahfstarttime RPC calls.

It is not allowed to update UAHF start time once the chain tip MTP exceeds
the existing configured start time (i.e. the fork is activated).

It is also not allowed to set it to a value less than 2 hours ahead of current
chain tip MTP.

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

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

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D243