RFR: 8280438: Improve BufferNode::Allocator::release to avoid walking pending list

[Kim Barrett]

Please review this change to BufferNode::Allocator::release to avoid needing
to walk the pending list in preparation for transferring it to the free list.
We need to have both the head and the tail of the pending list to perform that
transfer.

Rather than using a LockFreeStack to accumulate the pending list, we now use a
bespoke data structure that only supports concurrent additions, since that's
all we need.  This simplifies the code, and also makes it easy to capture the
first pushed node as the tail.

To make the transfer from the pending list to the free list, we double-buffer
the pending list.  Additions are made to the currently active pending list.  A
transfer flips which pending list is active, and transfers from the list that
was just deactivated.  Additions are made in a critical section.  The critical
section synchronization already performed when transferring now does double
duty, also synchronizing the pending list flip.

Testing:
mach5 tier1-3

Ran a test with -Xlog:gc,ptrqueue,freelist and verified transfers were
occurring as expected.

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Commit messages:
 - double-buffer pending list

Changes: https://git.openjdk.java.net/jdk/pull/7210/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7210&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280438
  Stats: 94 lines in 2 files changed: 55 ins; 7 del; 32 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7210.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7210/head:pull/7210

PR: https://git.openjdk.java.net/jdk/pull/7210