Hash field expiration is managed with two levels of data structures.
1. At the DB level, an ebuckets structure maintains the set of all
hashes that contain fields with expiration.
2. At the per-hash level, an ebuckets structure tracks fields with
expiration.
This pull request refactors the 1st level to operate per slot instead,
and introduces a new API called estore (expiration store). Its design
aligns closely with the existing kvstore API, ensuring consistency and
simplifying usage. The terminology at that level has been updated from
“HFE” or “hexpire” to “subexpiry”, reflecting a broader scope that can
later support other data types.
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