RFR: 8377142: Jtreg test gc/shenandoah/oom/TestThreadFailure.java triggers assert(young_reserve + reserve_for_mixed + reserve_for_promo <= old_available + young_available) failed

[Kelvin Nilsen]

In ShenandoahGenerationalHeap::compute_old_generation_balance(), adjust old_available and young_available when regions are transferred between young and old generations.

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Commit messages:
 - polish comments
 - Add comments to explain delicate balance between promo-reserve and mutation budget
 - Revert gratuitous formatting changes
 - Fix calculation of old_reserve when memory in short supply
 - Merge remote-tracking branch 'jdk/master' into fix-share-reserves-assert-error
 - Shrink old reserve when memory is in short supply
 - Add debug logs to help diagnose reproducer

Changes: https://git.openjdk.org/jdk/pull/29621/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29621&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377142
  Stats: 115 lines in 1 file changed: 60 ins; 31 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/29621.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29621/head:pull/29621

PR: https://git.openjdk.org/jdk/pull/29621