RFR: 8150134: Simplify concurrent refinement thread deactivation

[Kim Barrett]

Please review this simplification of the deactivation of concurrent
refinement threads.

I also took the opportunity to move the activate/deactivate logging
messages for those threads.  One benefit of this is that the primary
thread (worker_id 0) now generates such log messages.  Note that just
moving the logging within the activate/deactivate functions wouldn't
have fixed that, since the primary thread is not currently activated
via a call to the activate function.  That's an oddity that I plan to
fix later.

One side effect of the logging change: it is now apparent that the
primary refinement thread gets activated during a pause, though
fortunately immediately blocks on the SuspendibleThreadSetJoiner.
That's another problem that I plan to fix later.

CR:
https://bugs.openjdk.java.net/browse/JDK-8150134

Webrev:
http://cr.openjdk.java.net/~kbarrett/8150134/webrev.00/

Testing:
JPRT