RFR: Make region sampling aware of full collections

[William Kemper]

Full collections do not change the `_gc_state` of `ShenandoahHeap` in a way the region sampling code expected. The region sampling code is now aware of full collections.

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Commit messages:
 - Make region sampling aware of full collections

Changes: https://git.openjdk.java.net/shenandoah/pull/94/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=94&range=00
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/94.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/94/head:pull/94

PR: https://git.openjdk.java.net/shenandoah/pull/94