RFR: Generation resizing [v4]

[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.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 17 commits:

 - Merge branch 'shenandoah-master' into mmu-instrumentation
 - Remove vestigial lock, do not enroll periodic task while holding threads_lock
 - Remove unnecessary logging, clean up imports
 - Merge from shenandoah/master
 - 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
 - ... and 7 more: https://git.openjdk.org/shenandoah/compare/25469283...50896e31

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Changes: https://git.openjdk.org/shenandoah/pull/177/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=177&range=03
  Stats: 448 lines in 22 files changed: 418 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