RFR: Load balance remembered set scanning [v3]

[Kelvin Nilsen]

> This branch divides remembered set scanning into smaller units of work so that multiple cores can more effectively share the workload between them.  The benefit is to reduce concurrent scan remembered set times and to increase the parallelism of this phase.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Changes suggested by reviewer feedback

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/153/files
  - new: https://git.openjdk.org/shenandoah/pull/153/files/6b51cbf3..a806698e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=153&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=153&range=01-02

  Stats: 13 lines in 1 file changed: 6 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/shenandoah/pull/153.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/153/head:pull/153

PR: https://git.openjdk.org/shenandoah/pull/153