RFR: Reset coalesce and fill boundary during global collect

[William Kemper]

Normally, this happens during cset selection for the old generation. It also needs to happen for coalesce and fill at the end of global collections or objects may not be filled.

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Commit messages:
 - Reset coalesce and fill boundary during global collect

Changes: https://git.openjdk.java.net/shenandoah/pull/90/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=90&range=00
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/90.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/90/head:pull/90

PR: https://git.openjdk.java.net/shenandoah/pull/90