RFR (S) 8212178: Soft reference reclamation race in com.sun.xml.internal.stream.util.ThreadLocalBufferAllocator

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8212178

See the details in the bug. We are hitting this race when Shenandoah is running in aggressive mode
and evacuates lots of objects all the time, processing references very often. This makes weak
reference reclamation races very visible.

Fix:
  http://cr.openjdk.java.net/~shade/8212178/webrev.01/

I took the liberty of rewriting the entire method to make it more straightforward. New code
guarantees we never return "null" from this method.

Testing: failing tests, jdk:tier1, jdk-submit (running)

Thanks,
-Aleksey