Assume fixed size key (#10137)

Assume fixed size key (#10137)

Summary:
FastLRUCache now only supports 16B keys. The tests have changed to reflect this.

Because the unit tests were designed for caches that accept any string as keys, some tests are no longer compatible with FastLRUCache. We have disabled those for runs with FastLRUCache. (We could potentially change all tests to use 16B keys, but we don't because the cache public API does not require this.)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10137

Test Plan: make -j24 check

Reviewed By: gitbw95

Differential Revision: D37083934

Pulled By: guidotag

fbshipit-source-id: be1719cf5f8364a9a32bc4555bce1a0de3833b0d