RFR: 8319342: GenShen: Reset the count of degenerated cycles in a row following Full GC

[Kelvin Nilsen]

After a Full GC completes, reset the count on consecutive degenerated cycles to zero.  This prevents automatic triggering of back-to-back Full GC.

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Commit messages:
 - Do not increment degen count if degen upgrades to full
 - Count degen cycles in a row within ShenandoahCollectorPolicy

Changes: https://git.openjdk.org/shenandoah/pull/352/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=352&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319342
  Stats: 53 lines in 9 files changed: 16 ins; 32 del; 5 mod
  Patch: https://git.openjdk.org/shenandoah/pull/352.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/352/head:pull/352

PR: https://git.openjdk.org/shenandoah/pull/352