Summary:
... ahead of a larger change.
* Rename confusingly named `is_in_sec_cache` to `kept_in_sec_cache`
* Unify naming of "standalone" block cache entries (was "detached" in clock_cache)
* Remove some unused definitions in clock_cache.h (leftover from a previous revision)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11291

Test Plan: usual tests and CI, no behavior changes

Reviewed By: anand1976

Differential Revision: D43984642

Pulled By: pdillinger

fbshipit-source-id: b8bf0c5b90a932a88bcbdb413b2f256834aedf97