RFR: 8275035: Clean up worker thread infrastructure

[Per Liden]

I propose that we clean up our GangWorker/WorkGang and related classes, to remove abstractions we no longer need (after CMS was removed, MutexDispatcher was removed, Parallel is now using WorkGang, etc) and adjusting names as follows:

* Rename AbstractGangTask to WorkerTask
* Rename WorkGang to WorkerThreads
* Fold GangWorker into WorkerThread
* Fold WorkManager into WorkerThreads
* Move SubTaskDone and friends to a new workerUtils.hpp/cpp

I've split things up into several commits to make it easier to review.

Testing: Passes Tier 1-3 on all Oracle platforms.

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Commit messages:
 - Clean up naming in ReferenceProcessor
 - Clean up naming in Shenandoah
 - Clean up naming in HeapDumper/HeapInspector
 - Clean up naming in G1
 - Clean up naming in pretouch
 - Clean up naming in WeakProcessor
 - Clean up naming in ZGC
 - Remove unused log tag
 - Rename workgroup.hpp/cpp to workerThread.hpp/cpp
 - Clean up worker code
 - ... and 10 more: https://git.openjdk.java.net/jdk/compare/c032186b...39b23f42

Changes: https://git.openjdk.java.net/jdk/pull/5886/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5886&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275035
  Stats: 2415 lines in 105 files changed: 881 ins; 1083 del; 451 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5886.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5886/head:pull/5886

PR: https://git.openjdk.java.net/jdk/pull/5886