RFR: Enforce max regions [v2]

[Kelvin Nilsen]

> This commit enforces upper bounds on the number of ShenandoahHeapRegions affiliated with each generation.  Prior to this change, enforcement of generation sizes was by usage alone.  This allowed situations in which so many sparsely populated regions were affiliated with old-gen that there were insufficient FREE regions available to satisfy legitimate young-gen allocation requests.  This was resulting in excessive TLAB allocation failures and degenerated collections.

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

  Respond to reviewer feedback

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/179/files
  - new: https://git.openjdk.org/shenandoah/pull/179/files/f5b3e0db..28a53a86

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=179&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=179&range=00-01

  Stats: 60 lines in 4 files changed: 22 ins; 10 del; 28 mod
  Patch: https://git.openjdk.org/shenandoah/pull/179.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/179/head:pull/179

PR: https://git.openjdk.org/shenandoah/pull/179