RFR: Coalesce and fill objects immediately after final mark

[William Kemper]

Before this change, an allocation failure during a global concurrent cycle _after_ final mark would resume the degenerate cycle for evacuation. However, the degenerated cycle cleans up unmarked old objects during the marking phase. This scenario would leave pointers from unmarked objects into reclaimed memory and could lead to a crash when scanning the remembered set.

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Commit messages:
 - Coalesce and fill objects immediately after final mark

Changes: https://git.openjdk.java.net/shenandoah/pull/108/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=108&range=00
  Stats: 10 lines in 1 file changed: 6 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/108.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/108/head:pull/108

PR: https://git.openjdk.java.net/shenandoah/pull/108