TP.registerProbeHandler not thread safe

TP.registerProbeHandlers is not thread safe since it modifies preregistered_probe_handlers outside of any synchronization.

If a thread calls this method while another thread is inside startDiagnostics (which can happen easily with a shared transport), it can cause a NullPointerException when startDiagnostics is looping through preregistered_probe_handlers.

Access to preregistered_probe_handlers should be synchronized.