RFR: 8311883: [Genshen] Adaptive tenuring threshold [v5]

[Y. Srinivas Ramakrishna]

> JDK-8311883 [GenShen] Adaptive tenuring
> 
> I am opening this previously draft PR for formal preliminary review. It has already benefited from review feedback from a code walkthrough of an earlier version of the code. Most of that feedback and the corrections thereof are to be found in the comments in this PR. I have addressed a large majority of those comments, and am working on the last one that I plan to address as part of this PR. For the ones that I don't plan to address in this PR, I will create follow up tickets. Those will be added in the responses for the remaining feedback comments recorded in this PR's conversation.
> 
> Preliminary testing w/SPECjbb didn't yield reliable performance data from which to infer any performance improvements stemming from enabling adaptive tenuring. I believe that was because of the way SPECjbb is run, which causes excessive degenerate and full gc's. I plan to collect SPECjbb numbers with a fixed lower max HBIR so as to be able to discern performance differences from this change, as well as Extremem workloads. Those will be added here once ready over the next few days.

Y. Srinivas Ramakrishna has updated the pull request incrementally with one additional commit since the last revision:

  jcheck whitespace.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/289/files
  - new: https://git.openjdk.org/shenandoah/pull/289/files/a8dbbd66..4a1d6c8d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=289&range=04
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=289&range=03-04

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/289.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/289/head:pull/289

PR: https://git.openjdk.org/shenandoah/pull/289