Currently, if mocktime is set, the logs will be timestamped with the
mocktime instead of the system time. It's often useful to have the
system time in the logs (for example, when running an integration test,
it's useful to know what bitcoind was doing at system time so you can
see how it's interacting with the test framework or other bitcoind
nodes).

This PR timestamps the log with the actual system time, and also prints
out the mocktime if mocktime is being used.

./test/functional/test_runner.py --nocleanup rpc_uptime