Fix bug with kHashSearch and changing prefix_extractor with SetOptions (#10128)

Fix bug with kHashSearch and changing prefix_extractor with SetOptions (#10128)

Summary:
When opening an SST file created using index_type=kHashSearch,
the *current* prefix_extractor would be saved, and used with hash index
if the *new current* prefix_extractor at query time is compatible with
the SST file. This is a problem if the prefix_extractor at SST open time
is not compatible but SetOptions later changes (back) to one that is
compatible.

This change fixes that by using the known compatible (or missing) prefix
extractor we save for use with prefix filtering. Detail: I have moved the
InternalKeySliceTransform wrapper to avoid some indirection and remove
unnecessary fields.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10128

Test Plan:
expanded unit test (using some logic from https://github.com/facebook/rocksdb/issues/10122) that fails
before fix and probably covers some other previously uncovered cases.

Reviewed By: siying

Differential Revision: D36955738

Pulled By: pdillinger

fbshipit-source-id: 0c78a6b0d24054ef2f3cb237bf010c1c5589fb10