RFR: 8311978: Create abstraction over heap metrics for heuristics [v2]

[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:

  Fix zero builds

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14856/files
  - new: https://git.openjdk.org/jdk/pull/14856/files/8864183f..210604a3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14856&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14856&range=00-01

  Stats: 2 lines in 2 files changed: 2 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