RFR (S): 8189673: Consistent naming of concurrent threads, tasks and related identifiers

[Thomas Schatzl]

Hi,

  please review the following change that tries to consistently name
concurrent workers and related members to not contain any of "marking",
"mark", "parallel" or something similar, to use identifiers related to
"concurrent workers".

The reason is that G1's concurrent worker threads for a long time now
do more than strictly only "marking", but are also used for other, but
typically related tasks.

Also, do not use "parallel" to avoid confusion with the work gang used
during stw pauses. I think by now in the GC area the terms "concurrent"
and "parallel" have a sufficiently distinct meaning to be useful.

Also change _max_worker_id to the more specific _max_num_tasks to not
clash with the new naming. I.e. that it does not appear to be related
to the concurrent workers.

CR:
https://bugs.openjdk.java.net/browse/JDK-8189673
Webrev:
http://cr.openjdk.java.net/~tschatzl/8189673/webrev/
Testing:
jprt

Thanks,
  Thomas