RFR: Coalesce and fill old, pinned regions after old gen final mark [v4]

[William Kemper]

> Pinned regions were not being coalesced and filled. This fixes a crash observed with the specjbb 2015 benchmark.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 18 commits:

 - Prepare for review
 - Merge branch 'shenandoah-master' into fill-old-pinned
 - Validate old gen state transitions
 - Make use of old generation without down casting
 - Make verifier aware of changes to old generation state machine
 - Make rset scan aware of changes to old generation state machine
 - Fix use of index as count and over-counting of garbage
 - Sort out phase timing, gc sessions and add missing state transititions
 - Fix fastdebug compilation error
 - Slide pinned regions to the front of collection set candidates
 - ... and 8 more: https://git.openjdk.org/shenandoah/compare/16a9ab63...561e5e90

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Changes: https://git.openjdk.org/shenandoah/pull/149/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=149&range=03
  Stats: 1178 lines in 29 files changed: 741 ins; 274 del; 163 mod
  Patch: https://git.openjdk.org/shenandoah/pull/149.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/149/head:pull/149

PR: https://git.openjdk.org/shenandoah/pull/149