RFR: 8258254: Move PtrQueue flush to PtrQueueSet subclasses

[Kim Barrett]

Please review this change to the PtrQueue hierarchy, changing queue flushing
from an intrinsic operation of the queue to an operation the qset performs on
a queue.  This is a piece of the refactoring being done under JDK-8258251,
separated out for easier review.

This change also removes a couple of no longer used internal helper functions
from PtrQueue.

Testing:
mach5 tier1
local (linux-x64) hotspot:tier1 with -XX:+UseShenandoahGC

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Commit messages:
 - move flush

Changes: https://git.openjdk.java.net/jdk/pull/1851/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1851&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258254
  Stats: 135 lines in 12 files changed: 55 ins; 69 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1851.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1851/head:pull/1851

PR: https://git.openjdk.java.net/jdk/pull/1851