RFR: 8139200: Eliminate G1ParClosureSuper::_worker_id

[Kim Barrett]

Please review this cleanup of the construction of G1ParClosureSuper
and classes derived from it.

We move the _worker_id member from G1ParClosureSuper down to
G1ParCopyHelper, where it is actually used.  Other subclasses of
G1ParClosureSuper no longer need to worry about it.  In particular,
the complicated initialization of G1ParScanClosure is eliminated,
getting rid of G1ParClosureSuper::set_par_scan_thread_state.

[In G1ParCopyHelper we could obtain the worker_id from the
par_scan_state, but that would add a memory indirection in code that
is performance sensitive.]

CR:
https://bugs.openjdk.java.net/browse/JDK-8139200

Webrev:
http://cr.openjdk.java.net/~kbarrett/8139200/webrev.00/

Testing:
JPRT