RFR: 8342927: GenShen: Guarantee slices of time for coalesce and filling

[William Kemper]

By design, young collections may preempt old collections. However, in some cases, this leads to starvation of old collection cycles. When old generation collections cannot make progress, the system continues to get "backed up" until an eventual degenerated or full GC cleans out the old generation. Genshen already has a mechanism to guarantee progress for old marking. It should be extended to also provide for coalescing and filling.

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Commit messages:
 - Guarantee time slice for old marking and preparing for old marking

Changes: https://git.openjdk.org/shenandoah/pull/526/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=526&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342927
  Stats: 12 lines in 2 files changed: 3 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/shenandoah/pull/526.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/526/head:pull/526

PR: https://git.openjdk.org/shenandoah/pull/526