Stable cache keys on ingested SST files (#8669)

Stable cache keys on ingested SST files (#8669)

Summary:
Extends https://github.com/facebook/rocksdb/issues/8659 to work for ingested external SST files, even
the same file ingested into different DBs sharing a block cache.

Note: These new cache keys are currently only enabled when FileSystem
does not provide GetUniqueId. For now, they are typically larger,
so slightly less efficient.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8669

Test Plan: Extended unit test

Reviewed By: zhichao-cao

Differential Revision: D30398532

Pulled By: pdillinger

fbshipit-source-id: 1f13e2af4b8bfff5741953a69466e9589fbc23c7