RFR: 8318570: GenShen: Only start old cycles if a young cycle is ready to run

[William Kemper]

Each old cycle in the generational mode is preceded by a "bootstrap" young cycle. When old cycles are triggered independently of young cycles, the bootstrap cycle may be unproductive (especially if it follows a recent young cycle). With this change, the bootstrap cycle effectively replaces the ordinary young cycle.

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Commit messages:
 - Merge remote-tracking branch 'shenandoah/master' into subordinate-old-collections
 - Print heap region details at debug level
 - Transition to waiting-for-fill when mixed collections candidates are empty
 - Merge remote-tracking branch 'shenandoah/master' into subordinate-old-collections
 - Only start old cycles if it is time for a young cycle
 - Do not start a cycle when cause is `no_gc`
 - Fix misleading logging and double cancellation of old collections

Changes: https://git.openjdk.org/shenandoah/pull/344/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=344&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318570
  Stats: 38 lines in 6 files changed: 13 ins; 0 del; 25 mod
  Patch: https://git.openjdk.org/shenandoah/pull/344.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/344/head:pull/344

PR: https://git.openjdk.org/shenandoah/pull/344