RFR: Enable remembered set verification during global collections [v3]

[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 with a new target base due to a merge or a rebase. The pull request now contains 15 commits:

 - Merge branch 'shenandoah' into verify-rset-during-global-gc
 - Simplify object iteration and respond to review feedback.
 - Merge branch 'earthling-master' into verify-rset-during-global-gc
 - Merge branch 'shenandoah' into verify-rset-during-global-gc
 - Coalesce and fill old objects after final mark in a global collect
 - Only verify first object on next card when last object spans cards
 - Fix infinite loop scenario and invalid limit in verifier
 - Use safe offset when card runs past region top
 - Also use mark context when verifying rset at init mark for global collects
 - Do not read past top of region when looking for last object in card
 - ... and 5 more: https://git.openjdk.java.net/shenandoah/compare/ad15b6b5...7136de8c

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Changes: https://git.openjdk.java.net/shenandoah/pull/64/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=64&range=02
  Stats: 143 lines in 8 files changed: 60 ins; 44 del; 39 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