RFR: 8311978: Create abstraction over heap metrics for heuristics [v6]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'openjdk-master' into heap-stats-interface
 - Fix zero build
 - Have all heuristics use heap info interface
 - Put explicit constructor call back in
 - Fix zero builds
 - Create an abstraction over the heap for use with heuristics

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14856/files
  - new: https://git.openjdk.org/jdk/pull/14856/files/fb231a64..d4643280

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14856&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14856&range=04-05

  Stats: 20551 lines in 644 files changed: 12539 ins; 6860 del; 1152 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