RFR: Restore ShenandoahVerify functionality for generational mode [v2]

[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

earthling-amzn has updated the pull request incrementally with one additional commit since the last revision:

  Place '*' next to type, not variable
  
  Also remove unapproved 'override' keyword

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Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/24/files
  - new: https://git.openjdk.java.net/shenandoah/pull/24/files/b4d1ea78..c05fe2e9

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=24&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=24&range=00-01

  Stats: 27 lines in 11 files changed: 0 ins; 0 del; 27 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