RFR: 8258490: Shenandoah: Full GC does not need to remark threads and drain SATB buffers [v3]

[Zhengyu Gu]

> Full GC marks heap at a pause with SATB deactivated, therefore, we don't need to remark threads and drain SATB buffers during final mark phase.
> 
> - [x] hotspot_gc_shenandoah

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  @shade's comment

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1805/files
  - new: https://git.openjdk.java.net/jdk/pull/1805/files/602347da..acdb27e8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1805&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1805&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1805.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1805/head:pull/1805

PR: https://git.openjdk.java.net/jdk/pull/1805