RFR: 8314329: AgeTable: add is_clear() & allocation spec, and relax assert to allow use of 0-index slot [v4]

[Y. Srinivas Ramakrishna]

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

Y. Srinivas Ramakrishna has updated the pull request incrementally with one additional commit since the last revision:

  const-ed is_clear() method of AgeTable.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17470/files
  - new: https://git.openjdk.org/jdk/pull/17470/files/3572babf..676e8950

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17470&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17470&range=02-03

  Stats: 3 lines in 2 files changed: 0 ins; 0 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