RFR: Fix preempt old preparation

[Kelvin Nilsen]

To address crashes found following integration of "preempt old preparation", several fixes have been implemented:

1. Defer mixed evacuations until after coalesce-and-fill is done
2. Fix verification of remembered set to use the marking context whenever old-gen is in the process of preparing for mixed evacuations.
3. Cosmetic improvement to change "misleading" function names.

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Commit messages:
 - Improve function names
 - Merge branch 'shenandoah' of ssh://git.amazon.com/pkg/OpenJDKTipSrc into fix-preempt-old-preparation
 - Fix verification of remset to use mark context when appropriate
 - Defer mixed evacuations until after coalesce-and-fill is done
 - Merge branch 'shenandoah' of ssh://git.amazon.com/pkg/OpenJDKTipSrc into fix-preempt-old-preparation
 - Finish coalesce-and-fill before starting mixed evac

Changes: https://git.openjdk.java.net/shenandoah/pull/96/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=96&range=00
  Stats: 35 lines in 9 files changed: 14 ins; 8 del; 13 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/96.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/96/head:pull/96

PR: https://git.openjdk.java.net/shenandoah/pull/96