RFR(M): 8220293: Deadlock in JFR string pool

[Markus Gronlund]

Greeting,

Please review the following patch to address the following:

Bug: https://bugs.openjdk.java.net/browse/JDK-8220293
Webrev: http://cr.openjdk.java.net/~mgronlun/8220293/webrev01/ 

High level summary:

We don't want to release a retired buffer, because this will break the invariant that a successful acquisition implies a valid (non-retired) buffer.
This is the reason why this work is, at least prima facie, a little more involved.

Tests: 
test case / reproducer attached to: https://bugs.openjdk.java.net/browse/JDK-8220293 
open/test/jdk/:jdk_jfr

Thank you
Markus