RFR: 8312116: GenShen: make instantaneous allocation rate triggers more timely [v6]

[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 68 commits:

 - move some post_initialize() work into subclass ShenandoahGenerationalHeuristics
 - Merge remote-tracking branch 'jdk/master' into accelerated-triggers
 - Merge remote-tracking branch 'jdk/master' into accelerated-triggers
 - Fix comment
 - Use PROPERFMT macros
 - Simplify code flow: reviewer suggestion
 - Merge remote-tracking branch 'jdk/master' into accelerated-triggers
 - Remove develop/debug instrumentation
 - add another override
 - Change type of command-line args
 - ... and 58 more: https://git.openjdk.org/jdk/compare/49f72658...ea38ec15

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Changes: https://git.openjdk.org/jdk/pull/29039/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29039&range=05
  Stats: 1034 lines in 27 files changed: 927 ins; 35 del; 72 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