RFR: Split Generational Shenandoah memory pools

[David Alvarez]

Expose the Young Generation and the Old Generation as independent memory pools when running in generational mode

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Commit messages:
 - Split Generational Shenandoah memory pools

Changes: https://git.openjdk.java.net/shenandoah/pull/100/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=100&range=00
  Stats: 122 lines in 4 files changed: 110 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/100.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/100/head:pull/100

PR: https://git.openjdk.java.net/shenandoah/pull/100