Request for review: 7174978: NPG: Fix bactrace builder for class redefinition

[Coleen Phillimore]

Summary: Save the set of backtraces to use for on stack method walking 
for redefine classes.

I also moved metadataOnStackMark class to it's own file because it's not 
only used for redefine classes.   Some metadata can be individually 
deallocated (eg. the Method* created in the relocator).

open webrev at http://cr.openjdk.java.net/~coleenp/7174978/
bug link at http://bugs.sun.com/view_bug.do?bug_id=7174978

Ran test that will be added to the jdk/tests in 
java/lang/instrument/RedefineMethodInBacktrace.sh (to be checked in 
separately).

thanks,
Coleen