RFR(M): 8039147: Cleanup SuspendibleThreadSet

[Per Liden]

Hi,

Here's a attempt to cleanup SuspendibleThreadSet that I'd like to get 
some reviews on.

Webrev: http://cr.openjdk.java.net/~pliden/8039147/webrev.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8039147

Summary:

* The function of SuspendibleThreadSet is completely unchanged. Still 
does the same thing, same semantics, same users, etc.

* SuspendibleThreadSet is broken out from concurrentGCThread.hpp/cpp 
into separate files. The relation ship with ConcurrentGCThread is a bit 
artificial as SuspendibleThreadSet is used by other threads as well e.g. 
SharedHeap's FlexibleWorkGang.

* There's currently two ways of using SuspendibleThreadSet, depending on 
which context you're in; 1) through direct use of the static _sts member 
in ConcurrentGCThread and 2) via the static stsXXXX() functions exposed 
by ConcurrentGCThread. With this patch there's only one interface, which 
is exposed as static functions on the SuspendibleThreadSet class itself.

* I adjusted the names of some of the fields in SuspendibleThreadSet, 
which didn't quite make sense to me. For example, _async is now 
_nthreads since it's a counter of number of threads currently in the 
thread set.

* The should_yield() and yield() functions on ConcurrentGCThread (and 
it's subclasses) where removed, since they were never used. There was 
one exception here though, which was concurrentMarkThread::yield(), but 
it just does a direct call to SuspendibleThreadSet's yield() so that's 
just an useless indirection.

* SuspendibleThreadSetJoiner is introduced to replace code like this:

if (....) {
    _sts.join();
    <do something>
    _sts.leave();
}

with code like this:

if (....) {
    SuspendibleThreadSetJoiner sts;
    <do something>
}

cheers,
/Per