RFR: Coalesce and fill old, pinned regions after old gen final mark [v3]

[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 eight commits:

 - WIP: Slide pinned regions to the front of collection set candidates
   
   There's an off-by-one error here that needs to be fixed.
 - Merge branch 'shenandoah-master' into fill-old-pinned
 - WIP: Add a "unit" test for exercising old pinned region code paths
 - WIP: Start changing mixed collections to handle pinned old regions
 - WIP: Restore ability to interrupt making old regions parseable
 - WIP: Factor filling old out of collection cycle. It needs to happen before bitmap is reset
 - remove TODO comment
 - Coalesce and fill old, pinned regions after old gen final mark

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Changes: https://git.openjdk.org/shenandoah/pull/149/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=149&range=02
  Stats: 995 lines in 23 files changed: 557 ins; 299 del; 139 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