RFR: 8213352: Separate BufferNode allocation from PtrQueueSet

[Kim Barrett]

Please review this change to PtrQueue buffer allocation, moving the
implementation of free-list based allocation from PtrQueueSet to a
separate BufferNode::Allocator object.  The old code supported sharing
of free-lists by having one PtrQueueSet delegate to another; now
free-list sharing is provided by using the same allocator object.

CR:
https://bugs.openjdk.java.net/browse/JDK-8213352

Webrev:
http://cr.openjdk.java.net/~kbarrett/8213352/open.00/

Testing:
mach5 tier1-5
Added gtest for new allocator class.