RFR (S) 8173658: JvmtiExport::post_class_unload() is broken for non-JavaThread initiators

[coleen . phillimore]

Summary: call extension ClassUnload event as a deferred event from the 
ServiceThread and remove unsafe arguments

I'm still waiting for the CSR request to get approved but this change 
fixes the broken class unload events.  It's been tested with the 
existing test case, and hs-tier1 for all platforms and tier2-6 on 
linux-x64-debug.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8173658.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8173658

Thanks,
Coleen