RFR: 8368152: Shenandoah: Incorrect behavior at end of degenerated cycle

[William Kemper]

There are several issues addressed in this PR:
* Shenandoah always ran a full GC after any degenerated cycle
* The number of consecutive degenerated GCs with bad progress was reset for every degenerated cycle
* Good progress was reported in generational mode even when no progress is made

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Commit messages:
 - Improve documentation, add unit test
 - Fix logic for handling degenerated cycles that do not make good progress

Changes: https://git.openjdk.org/jdk/pull/27456/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27456&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368152
  Stats: 123 lines in 5 files changed: 101 ins; 9 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/27456.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27456/head:pull/27456

PR: https://git.openjdk.org/jdk/pull/27456