RFR (M) 8164921: Memory leaked when instrumentation.retransformClasses() is called repeatedly

[Coleen Phillimore]

Summary: Return Metablocks smaller than dictionary's dark matter.

This change contributed by Jon Masamitsu and myself.  To reclaim "dark 
matter" this change adds an array of small blocks by size, created 
lazily, to return Metablocks smaller than the BinaryTreeDictionary 
entry's minimum size.   This change also fixed a bug in small object 
double free and adds debugging code to check for this case.

With this change, the submitted test case runs indefinitely.   Also 
passed rbt tier 1-5 testing.

open webrev at http://cr.openjdk.java.net/~coleenp/8164921.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8164921

Thanks,
Coleen and Jon