RFR: Improve humongous remset scan [v2]

[Kelvin Nilsen]

> These changes allow humongous objects residing within the remembered set to be scanned by muitiple concurrent GC worker threads, allowing more efficient parallelization of the remembered set scanning effort.

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

  Improvements requested by reviewer

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/144/files
  - new: https://git.openjdk.org/shenandoah/pull/144/files/3b558e0a..0d7388ad

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=144&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=144&range=00-01

  Stats: 13 lines in 2 files changed: 0 ins; 11 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/144.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/144/head:pull/144

PR: https://git.openjdk.org/shenandoah/pull/144