Testing infrastructure: mocktime fixes

New, undocumented-on-purpose -mocktime=timestamp command-line
argument to startup with mocktime set. Needed because
time-related blockchain sanity checks are done on startup, before a
test has a chance to make a setmocktime RPC call.

And changed the setmocktime RPC call so calling it will not result in
currently connected peers being disconnected due to inactivity timeouts.