RFR: 8221394: Clean up ConcurrentGCThread

[Per Liden]

ConcurrentGCThread currently uses Terminator_lock to coordinate 
shutdown. This is dubious, as this lock is shared by many threads, some 
of which calls notify() rather than notify_all() to signal that 
something happened. It's also potentially inefficient as we might be 
waking up more threads than needed. Each ConcurrentGCThread should have 
it's own monitor for this.

This patch introduces a separate monitor for each instance of 
ConcurrentGCThread and does some general code cleanups in that class.

Bug: https://bugs.openjdk.java.net/browse/JDK-8221394
Webrev: http://cr.openjdk.java.net/~pliden/8221394/webrev.0

/Per