RFR: 8255984: Shenandoah: "adaptive" heuristic is prone to missing load spikes [v2]

[earthling-amzn]

> This change adds a "reactive" heuristic for triggering concurrent GC cycles.
> 
> The reactive heuristic maintains a margin of error and an allocation spike detection mechanism to trigger cycles somewhat more aggressively than the 'adaptive' heuristic. This heuristic 'reacts' to the outcome of GC cycles by adjusting the sensitivity of the triggers.
> 
> JBS ticket is here: https://bugs.openjdk.java.net/browse/JDK-8255984
> 
> The "adaptive" heuristic remains the default.
> 
> Steps to reproduce and test will follow shortly (there are no new jtreg test failures for Shenandoah with this change).

earthling-amzn has updated the pull request incrementally with two additional commits since the last revision:

 - Remove const qualifier from should_start_gc
   
   This lets the heuristics update state without const_casts.
 - Rename ShenandoahReactive flags to ShenandoahAdaptive flags.

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1099/files
  - new: https://git.openjdk.java.net/jdk/pull/1099/files/7f1de19b..275e80a2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1099&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1099&range=00-01

  Stats: 31 lines in 13 files changed: 0 ins; 2 del; 29 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1099.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1099/head:pull/1099

PR: https://git.openjdk.java.net/jdk/pull/1099