RFR: 8311978: Create abstraction over heap metrics for heuristics [v7]

[William Kemper]

> This change introduces a new interface `ShenandoahHeapStats`. Presently, the interface is implemented by `ShenandoahHeap`. The generational mode for Shenandoah will provide a new implementation of this interface `ShenandoahGeneration`.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Add documentation describing each heuristic

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14856/files
  - new: https://git.openjdk.org/jdk/pull/14856/files/d4643280..8192e33f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14856&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14856&range=05-06

  Stats: 45 lines in 7 files changed: 45 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14856.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14856/head:pull/14856

PR: https://git.openjdk.org/jdk/pull/14856