RFR: Enable remembered set verification during global collections [v2]

[William Kemper]

> There are two sets of changes here:
>  * Fixes to the verifier itself to enable remembered set verification during global collection
>  * Fixes to prevent the remembered set scan from parsing unmarked objects after a global collection
> 
> These changes pass the dacapo suite with `-XX:+ShenandoahVerify` and pass the shenandoah tier1 tests.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Simplify object iteration and respond to review feedback.

-------------

Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/64/files
  - new: https://git.openjdk.java.net/shenandoah/pull/64/files/d75a315d..c9c23a44

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=64&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=64&range=00-01

  Stats: 38 lines in 2 files changed: 3 ins; 19 del; 16 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/64.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/64/head:pull/64

PR: https://git.openjdk.java.net/shenandoah/pull/64