RFR(s): 8079579: Add SuspendibleThreadSetLeaver and make SuspendibleThreadSet::joint()/leave() private

[Per Liden]

Hi,

This patch adds a SuspendibleThreadSetLeaver (to accompany the 
SuspendibleThreadSetJoiner), and get rid off all direct calls to 
SuspendibleThreadSet::joint()/leave(). To cater for all use cases the 
existing SuspendibleThreadSetJoiner also needed a change, with an added 
argument to the constructor to allow the joiner/leaver to be disabled.

The patch also does a number of renames of the sts variable to sts_join, 
to make it distinct from the new sts_leave.

Webrev: http://cr.openjdk.java.net/~pliden/8079579/webrev.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8079579

Testing: jprt, gc-test-suite

PS. I've had this patch sitting in a local repo for quite some time. 
Bengt and I have discussed other enhancements in this area, but I my 
time has been taken up by other things. Instead of letting this patch 
rot I suggest we do this enhancement as a first step.

cheers,
/Per