RFR: Generation resizing

[William Kemper]

These changes have the generational mode track the minimum mutator utilization (percentage of process time used by mutators). When it falls below a configuration percentage (GCTimeRatio), a heuristic will transfer memory capacity to whatever generation has been using more CPU time. The assumption here is that by increasing capacity, we will decrease the collection frequency and improve the MMU.

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Commit messages:
 - Document the class responsible for adjusting generation sizes
 - Revert unnecessary change
 - Remove unused time between cycle tracking
 - Remove vestigial mmu tracker instance
 - Clamp adjustments to min/max when increment is too large
 - Adjust generation sizes from safepoint
 - Fix crash in SATB mode, always log average MMU on scheduled interval
 - Limits on generation size adjustments, log young/old heap occupancy in generational mode
 - WIP: Transfer up to 10% capacity to undersized generation
 - WIP: Track idle gc time and mmu averages, rename confusing method name
 - ... and 3 more: https://git.openjdk.org/shenandoah/compare/998f68b2...b916a909

Changes: https://git.openjdk.org/shenandoah/pull/177/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=177&range=00
  Stats: 449 lines in 22 files changed: 419 ins; 18 del; 12 mod
  Patch: https://git.openjdk.org/shenandoah/pull/177.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/177/head:pull/177

PR: https://git.openjdk.org/shenandoah/pull/177