JFR: Add JavaThreadStatistics event

[tajila]

Add support for the JavaThreadStatistics event described here, https://sap.github.io/SapMachine/jfrevents/21.html#javathreadstatistics

This event is profiled and is triggered every second, so it will require the use of the VM JFR profiler thread (see jfrSamplingThreadProc)

A new function jfrThreadStatistics can be added which is called every second to collect the relevant thread statistics.

Java thread creation is done in vmthread.cpp in startJavaThreadInternal.

Daemon thread can be determined by looking at vmthread->privateFlags & J9_PRIVATE_FLAGS_DAEMON_THREAD

We will need to add a U_64 peakThreads, U_64 liveThreads and U_64 cumulativeThreads fields to J9JavaVM to keep track of those. THe updates to those values can be performed when the vm->vmThreadListMutex is held.

[tajila]

@adpopescu Please take a look at this.