[jdk16u] RFR: 8258490: Shenandoah: Full GC does not need to remark threads and drain SATB buffers

[Aleksey Shipilev]

This fixes a minor Shenandoah inefficiency and allows clean backports of other fixes.

-------------

Commit messages:
 - Backport f80c63b38035f6b6969cca08cfc534d0476105af

Changes: https://git.openjdk.java.net/jdk16u/pull/52/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16u&pr=52&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258490
  Stats: 44 lines in 1 file changed: 21 ins; 15 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk16u/pull/52.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16u pull/52/head:pull/52

PR: https://git.openjdk.java.net/jdk16u/pull/52