RFR 8055008: Clean up code that saves the previous versions of redefined classes

[Coleen Phillimore]

Summary: Use scratch_class to find EMCP methods for breakpoints if the 
old methods are still running

See bug for more details.  This fix also includes a change to 
nmethod::metadata_do() to not walk the _method multiple times (the 
_method is added to the metadata section of the nmethod), and an attempt 
to help the sweeper clean up these scratch_class instance classes sooner.

Tested with nsk tests, java/lang/instrument tests and jck tests (which 
include some jvmti tests).

open webrev at http://cr.openjdk.java.net/~coleenp/8055008/
bug link https://bugs.openjdk.java.net/browse/JDK-8055008

Thanks,
Coleen