Summary:
ClockCache is still in experimental stage, and currently fails some pre-release fbcode tests. See https://www.internalfb.com/diff/D37772011. API calls to construct ClockCache are done via the function NewClockCache. For now, NewClockCache calls will return an LRUCache (with appropriate arguments), which is stable.

The idea that NewClockCache returns nullptr was also floated, but this would be interpreted as unsupported cache, and a default LRUCache would be constructed instead, potentially causing a performance regression that is harder to identify.

A new version of the NewClockCache function was created for our internal tests.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10351

Test Plan: ``make -j24 check`` and re-run the pre-release tests.

Reviewed By: pdillinger

Differential Revision: D37802685

Pulled By: guidotag

fbshipit-source-id: 0a8d10612ff21e576f7360cb13e20bc36e244972