RFR: 8372250: Merge PtrQueue into SATBMarkQueue [v3]

[Ben Taylor]

> This change merges `PtrQueue` into its only child class, `SATBMarkQueue`. The corresponding `PtrQueueSet` is merged with `SATBMarkQueueSet`, and `G1SATBMarkQueueSet` and `ShenandoahSATBMarkQueueSet` continue to inherit from that class.
> 
> There are few code modifications - most methods were moved directly from `PtrQueue` to `SATBMarkQueue`. The one meaningful change is to  preserve `SATBMarkQueue`'s call to `filter(queue);` before `flush_queue`. 
> 
> This change passes all tier1 jtreg tests with both Shenandoah and G1 in local testing.

Ben Taylor has updated the pull request incrementally with one additional commit since the last revision:

  Remove virtual from SATBMarkQueueSet::enqueue_completed_buffer

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28755/files
  - new: https://git.openjdk.org/jdk/pull/28755/files/148aa4f8..517b245f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28755&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28755&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28755.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28755/head:pull/28755

PR: https://git.openjdk.org/jdk/pull/28755