RFR: Restore ShenandoahVerify functionality for generational mode

[earthling-amzn]

### Summary
1. Add remembered set to verifier roots for young generation collections
2. Don't follow pointers outside the generation during verify traversals
3. Verify generation's `used` value
4. Correct errors in computing a generation's `used` values

### Testing
1. Dacapo suite completes without verification errors
2. hotspot_gc_shenandoah completes without verification errors

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Commit messages:
 - Fix young generation usage when humongous objects are promoted
 - Fix copy paste error accounting for humongous region usage
 - Count actual size of humongous objects towards generation usage
 - Only include rset in roots for young generation
 - Verify regions and oops in the active generation
 - Include remembered set in verification roots
 - Verify generation usage and region states

Changes: https://git.openjdk.java.net/shenandoah/pull/24/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=24&range=00
  Stats: 176 lines in 15 files changed: 154 ins; 6 del; 16 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/24.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/24/head:pull/24

PR: https://git.openjdk.java.net/shenandoah/pull/24