RFR (L) 8061205: MetadataOnStackMark only needs to walk code cache during class redefinition

[Coleen Phillimore]

Summary: Only do full metadata walk during class redefinition and only 
walk handles during class unloading.

This change decouples metadata walking for redefinition and class 
unloading, so that class unloading for G1 doesn't walk the code cache.  
It also decouples GC and on_stack marking in the code cache.

open webrev at http://cr.openjdk.java.net/~coleenp/8061205/
bug link https://bugs.openjdk.java.net/browse/JDK-8061205

Tested with FMW performance runs. vm.quick.testlist, 
jdk/test/java/lang/instrument tests and JPRT.

Thanks,
Coleen