RFR (S): 8133047: Rename G1ParScanThreadState::_queue_num to _worker_id

[Thomas Schatzl]

Hi all,

  can I have reviews for this small cleanup that renames the
G1ParScanThreadState::_queue_num member to worker_id like it is called
for all members that have the same functionality.

G1ParScanThreadState::_queue_num is actually compared to a worker_id
member of another class, and assigned to a _worker_id member somewhere
else in the code.

It hopefully makes the code slightly easier to read in the future by not
using different names for the same thing throughput the code.

CR:
https://bugs.openjdk.java.net/browse/JDK-8133047
Webrev:
http://cr.openjdk.java.net/~tschatzl/8133047/webrev/
testing:
jprt

Thanks,
  Thomas