RFR: Load balance remembered set scanning

[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.

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Commit messages:
 - Fix white space
 - Load balance remembered set scanning

Changes: https://git.openjdk.org/shenandoah/pull/153/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=153&range=00
  Stats: 649 lines in 8 files changed: 543 ins; 54 del; 52 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