move traceMask calculation logic into the trace log guard

Similar to ZOOKEEPER-3708 and ZOOKEEPER-3728 in Learner,  I found four cases where traceMask calculation logic are out of trace log guard in LearnerHandler, PrepRequestProcessor, and ReadOnlyRequestProcessor. The traceMask only matters if trace is enabled, so move the associated code into the logging guard. The fix is simple and I will create a PR later.