RFR: 8327097: GenShen: Align PLAB sizes down rather than up [v13]

[Kelvin Nilsen]

> When adjusting LAB sizes, round down rather than rounding up.  Otherwise, we may violate the ShenandoahHumongousThreshold bound.

Kelvin Nilsen has updated the pull request incrementally with four additional commits since the last revision:

 - Add a missing include to fix zero build
 - Add TODO comment for future refactoring of _plab initialization
 - Bug fixes to address regressions encountered during integration testing
 - Tighten up assert to assist with debugging

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/401/files
  - new: https://git.openjdk.org/shenandoah/pull/401/files/ecd6615f..71d9dced

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=12
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=11-12

  Stats: 27 lines in 7 files changed: 13 ins; 3 del; 11 mod
  Patch: https://git.openjdk.org/shenandoah/pull/401.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/401/head:pull/401

PR: https://git.openjdk.org/shenandoah/pull/401