RFR: Update heap usage tallies for generations after full GC compaction

[William Kemper]

Two changes here:
* Verification checks heap usage for _each_ generation every time (rather than only check the _active_ generation).
* Compute heap usage after post compaction phase of full gc (rather than pre-compaction phase).

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Commit messages:
 - Simplify live usage accounting for generations during full gc
 - Verify usage tallies for all generations during verification

Changes: https://git.openjdk.java.net/shenandoah/pull/84/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=84&range=00
  Stats: 149 lines in 2 files changed: 57 ins; 83 del; 9 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/84.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/84/head:pull/84

PR: https://git.openjdk.java.net/shenandoah/pull/84