RFR: Reset coalesce and fill boundary during global collect [v2]

[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.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Add comment explaining why coalesce and fill boundary needs to be reset here

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Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/90/files
  - new: https://git.openjdk.java.net/shenandoah/pull/90/files/50d110ba..7d590284

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=90&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=90&range=00-01

  Stats: 4 lines in 1 file changed: 4 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