RFR(S): 8210024: JFR calls virtual is_Java_thread from ~Thread()

[Markus Gronlund]

Greetings,

Please review the following changes to help make the JFR deregistration / buffer release steps a bit more robust as thread exits.

There is a need to release the JNIHandle for the EventWriter while the JavaThread is still on the threads list, since there is a sensitive interplay with the G1-prebarrier, and this change will help assist in the removal of shared PtrQueues.

The virtual is_Java_Thread() call can still be invoked for the NonJavaThreads in their destructor, but since the thread is still a NonJavaThread at this point, it should be possible to make is_Java_thread() pure virtual in Thread.
I did not find any proper non-platform specific exit location for NonJavaThreads, so they release as part of the destructor. 

Bug: https://bugs.openjdk.java.net/browse/JDK-8210024 
Webrev: http://cr.openjdk.java.net/~mgronlun/8210024/webrev01/ 
Testing: JFR tests, tier1, tier2, tier3

Comments:
The JfrThreadSampler changes should have been done when the JfrThreadSampler thread was turned into a NonJavaThread - removing the dead export code as part of this change.
The Jfr::on_java_thread_dismantle() is an intermediary support that will be removed when we change overall thread registration from on_exit() to on_start(), which are planned upcoming changes.

Thanks to Kim for detailing several involved issues.

Thanks
Markus