RFR: 8314329: AgeTable: add is_clear() & allocation spec, and relax assert to allow use of 0-index slot

[Y. Srinivas Ramakrishna]

I'd like to propose the following changes to the AgeTable (motivated by the uses in generational shenandoah):

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Commit messages:
 - JDK-8314329 AgeTable: add is_clear() & allocation spec, and relax assert

Changes: https://git.openjdk.org/jdk/pull/17470/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17470&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314329
  Stats: 17 lines in 2 files changed: 13 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/17470.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17470/head:pull/17470

PR: https://git.openjdk.org/jdk/pull/17470