RFR: 8312116: GenShen: make instantaneous allocation rate triggers more timely [v31]

[Kelvin Nilsen]

> After studying large numbers of GC logs with degenerated cycles that have resulted from "late" triggers, we propose the following general improvements:
> 
> 1. Track trends in GC times rather than always using the average GC time plus standard deviation.  In many situations, GC times trend upward due to, for example, increasing amounts of live data that must be marked as a workload builds up its working set of memory.
> 2. Sample allocation rates more frequently than once every 100 ms.
> 3. Track trends in allocation rates.  In some situations, the allocation rate trends upwards due to, for example, the start of a new phase of execution or a spike in client workload.
> 4. When we detect acceleration of allocation rate, predict consumption of memory based on accelerated allocation rates rather than assuming constant allocation rate.

Kelvin Nilsen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 105 commits:

 - Merge remote-tracking branch 'jdk/master' into accelerated-triggers
 - remove _is_generational, _regulator_thread, _control_thread from ShenandoahAdaptiveHeuristics
 - Remove resume_idle_span()
 - Remove arg to compute_headroom_adjustment() and update headroom if soft_capacity is managed
 - ShenandoahAccelertaionSamplePeriod is measured in ms
 - Represent ShenandoahAccelerationSamplePeriod in ms and require all adaptive triggers to wait for some garbage to accumulate
 - Improve formatting and comments
 - Improve comments
 - Remove dead code and unused variables and rename one function
 - fix spelling errors in comment
 - ... and 95 more: https://git.openjdk.org/jdk/compare/c8338be9...a20e69f5

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Changes: https://git.openjdk.org/jdk/pull/29039/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29039&range=30
  Stats: 937 lines in 24 files changed: 827 ins; 37 del; 73 mod
  Patch: https://git.openjdk.org/jdk/pull/29039.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29039/head:pull/29039

PR: https://git.openjdk.org/jdk/pull/29039