Mark -uahfstarttime parameter as deprecated and issue a log warning if set to anything other than default

Summary: As per Thomas Zander's suggestion, this parameter should ideally be disabled altogether. However we can't really easily get rid of it as a lot of our python rpc tests make use of it to selectively test pre and post fork behavior. So, I decided to just say in the help that this parameter is deprecated and print a log warning if it's set to anything other than the default. This way perhaps if some day in the future we get a bug report, we can at least see in the log if they forced UAHF activation to some non-standard time.

Test Plan:

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

Also try running ...

bitcoind -uahfstarttime=12345 -printtoconsole

... and look at the log messages. Should get a warning.

Reviewers: freetrader, deadalnix, zander, schancel, #bitcoin_abc

Reviewed By: freetrader, zander, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D477