Limit trace map size in TraceMonitor

The org.hippoecm.frontend.model.TraceMonitor accumulates all the loaded JCR item info (pairs of item#toString() and stack trace!) in static map when debug level logging turned on.
This can easily lead to OOME if a project turns on debug level logging.
I assume the TraceMonitor is kind of facility for developer (normally on single-user testing environment), so I think it should have a limited size map instead of no-limit map now.