RFR: Collect old humongous regions immediately at end of concurrent old mark

[William Kemper]

Recycle dead humongous objects at the end of concurrent old mark, rather than add them to the cset. This behavior is similar to young gen cset selection. It also increases the amount of free memory in old gen to support evacuations. Shenandoah has a specific code path for recycling humongous objects and (debug builds) will assert when an old region is added to the cset.

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Commit messages:
 - Log the number of regions trashed when humongous object is reclaimed
 - Recycle old humongous objects immediately at the end of old marking

Changes: https://git.openjdk.java.net/shenandoah/pull/44/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=44&range=00
  Stats: 57 lines in 3 files changed: 3 ins; 34 del; 20 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/44.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/44/head:pull/44

PR: https://git.openjdk.java.net/shenandoah/pull/44