RFR (M) 8198313: Wrap holder object for ClassLoaderData in a WeakHandle

[coleen.phillimore at oracle.com]

Summary: Use WeakHandle for ClassLoaderData::_holder so that is_alive 
closure is not needed

The purpose of WeakHandle is to encapsulate weak oops within the runtime 
code in the vm.  The class was initially written by StefanK.   The weak 
handles are pointers to OopStorage.   This code is a basis for future 
work to move direct pointers to the heap (oops) from runtime structures 
like the StringTable, into pointers into an area that the GC efficiently 
manages, in parallel and/or concurrently.

Tested with mach5 tier 1-5.  Performance tested with internal dev-submit 
performance tests, and locally.

open webrev at http://cr.openjdk.java.net/~coleenp/8198313.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8198313

Thanks,
Coleen