RFR: 8227269: Slow class loading when running JVM in debug mode

[Roman Kennke]

Hello all,

Issue:
https://bugs.openjdk.java.net/browse/JDK-8227269

I am proposing what amounts to a rewrite of classTrack.c. It avoids 
throwing away the class cache on GC, and instead keeps track of 
loaded/unloaded classes one-by-one.

In addition to that, it avoids this whole dance until an agent registers 
interest in EI_GC_FINISH.

Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8227269/webrev.01/

Testing: manual testing of provided test scenarios and timing.

Eg with the testcase provided here:
https://bugzilla.redhat.com/show_bug.cgi?id=1751985

I am getting those numbers:
unpatched: no debug: 84s with debug: 225s
patched:   no debug: 85s with debug: 95s

I also tested successfully through jdk/submit repo

Can I please get a review?

Thanks,
Roman