RFR: Coalesce and fill old, pinned regions after old gen final mark [v2]

[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 incrementally with four additional commits since the last revision:

 - 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

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/149/files
  - new: https://git.openjdk.org/shenandoah/pull/149/files/cfb672d3..ade000aa

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=149&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=149&range=00-01

  Stats: 845 lines in 23 files changed: 444 ins; 261 del; 140 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