RFR: 8258252: Move PtrQueue enqueue to PtrQueueSet subclasses

[Kim Barrett]

Please review this refactoring in the PtrQueue/PtrQueueSet hierarchy.

The functions PtrQueue::enqueue and PtrQueue::enqueue_known_active are
removed, along with the helper virtual function PtrQueue::handle_completed_buffer.

Instead we now have G1{Dirty,Redirty}CardQueueSet::enqueue and SATBMarkQueueSet::enqueue/enqueue_known_active, each tailored to the corresponding specific behavior. They use some shared helper routines from PtrQueueSet, and the PtrQueue buffer and index have been made accessible to clients.

Testing:
mach5 tier1-3, 5 (stress tests)
Local (linux-x64) tier1 with -XX:+UseShenandoahGC

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Commit messages:
 - shenandoah support
 - move enqueue to qsets

Changes: https://git.openjdk.java.net/jdk/pull/1776/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1776&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258252
  Stats: 364 lines in 22 files changed: 181 ins; 113 del; 70 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1776.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1776/head:pull/1776

PR: https://git.openjdk.java.net/jdk/pull/1776