RFR: 8311978: Create abstraction over heap metrics for heuristics

[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`.

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Commit messages:
 - Create an abstraction over the heap for use with heuristics

Changes: https://git.openjdk.org/jdk/pull/14856/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14856&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311978
  Stats: 63 lines in 7 files changed: 48 ins; 1 del; 14 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