RFR: 8151670: Unexpected concurrent refinement deactivation and reactivation

[Kim Barrett]

Please review this change to the concurrent refinement threads to use
SuspendibleThreadSet::yield, rather than deactivation, in response to
an STS suspension request. This should mostly reduce the cost of
coming out of a non-GC safepoint. See the CR for further discussion.

CR:
https://bugs.openjdk.java.net/browse/JDK-8151670

Webrev:
http://cr.openjdk.java.net/~kbarrett/8151670/webrev.00/

Testing:
JPRT, RBT GC Nightly, local specjbb2015.