RFR: 8339960: GenShen: Fix inconsistencies in generational Shenandoah behavior

[Kelvin Nilsen]

This fixes some bugs found in recent code review and playback of an assertion failure.

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Commit messages:
 - Use -1 to represent rightmost for empty partitions in find_regions_with_alloc_capacity()
 - Report generation resize and clear region_balance in ShenFreeSet::finish_rebuild()
 - Check available rather than capacity before logging insufficient reserves
 - initialize rightmosts to -1 in find_regions_with_alloc_capacity()
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - Merge branch 'openjdk:master' into master
 - ... and 16 more: https://git.openjdk.org/shenandoah/compare/662d146d...e78c7b0d

Changes: https://git.openjdk.org/shenandoah/pull/497/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=497&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339960
  Stats: 28 lines in 1 file changed: 20 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/shenandoah/pull/497.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/497/head:pull/497

PR: https://git.openjdk.org/shenandoah/pull/497