RFR: Recompute generation accounts after fullgc

[Kelvin Nilsen]

This fix addresses a problem found during verification following full gcs.  This recomputes the generation usage at the end of full GC as part of a 5th phase of full gc.

We may be able to remove this 5th phase at a later time after we find and fix whatever errors exist in generation accounting in the existing full gc implementation.

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Commit messages:
 - Merge remote-tracking branch 'GitFarmBranch/recompute-generation-accounts-after-fullgc' into recompute-generation-accounts-after-fullgc
 - Recompute generation usage following Full GC

Changes: https://git.openjdk.org/shenandoah/pull/235/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=235&range=00
  Stats: 72 lines in 4 files changed: 72 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/235.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/235/head:pull/235

PR: https://git.openjdk.org/shenandoah/pull/235