RFR (M) 8212160: JVMTI agent crashes with "assert(_value != 0LL) failed: resolving NULL _value"

[coleen.phillimore at oracle.com]

Summary: Add local deferred event list to thread to post events outside 
CodeCache_lock.

This patch builds on the patch for JDK-8173361.  With this patch, I made 
the JvmtiDeferredEventQueue an instance class (not AllStatic) and have 
one per thread.  The CodeBlob event that used to drop the CodeCache_lock 
and raced with the sweeper thread, adds the events it wants to post to 
its thread local list, and processes it outside the lock.  The list is 
walked in GC and by the sweeper to keep the nmethods from being unloaded 
and zombied, respectively.

Also, the jmethod_id field in nmethod was only used as a boolean so 
don't create a jmethod_id until needed for post_compiled_method_unload.

Ran hs tier1-8 on linux-x64-debug and the stress test that crashed in 
the original bug report.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8212160.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8212160

Thanks,
Coleen