RFR: Coalesce dead objects in pinned old regions during full gc

[William Kemper]

Pinned regions are not compacted during a full gc. For old regions, this may leave references in unmarked objects pointing into relocated or reclaimed memory. These unmarked objects need to be filled in to prevent the remembered set scan from following bad pointers.

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Commit messages:
 - Coalesce and fill pinned regions during update reference phase of full gc
 - Make post compaction step multi threaded
 - Coalesce and fill pinned old regions during full gc

Changes: https://git.openjdk.java.net/shenandoah/pull/116/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=116&range=00
  Stats: 7 lines in 1 file changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/116.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/116/head:pull/116

PR: https://git.openjdk.java.net/shenandoah/pull/116