[jdk11u-dev] RFR: 8261496: Shenandoah: reconsider pacing updates memory ordering

[Aleksey Shipilev]

This backport does not apply cleanly, because the `Atomic` parameter orders had changed since [JDK-8234563](https://bugs.openjdk.java.net/browse/JDK-8234563). Passes `hotspot_gc_shenandoah`.

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Commit messages:
 - Backport 4642730b057bfa37df740fc8d9e17bfb60e53447

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/66/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=66&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261496
  Stats: 7 lines in 3 files changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/66.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/66/head:pull/66

PR: https://git.openjdk.java.net/jdk11u-dev/pull/66