Summary:
The new API in https://github.com/facebook/rocksdb/issues/13453 is awkward and precarious because of using RangePtr, which encodes optional keys using raw pointers to Slice. We could use `std::optional<Slice>` instead but that is unsatisfyingly a larger object with an inefficient size (typically 17 bytes).
Here I introduce a custom optional Slice type, `OptSlice`, that is the same size as a Slice, and use it in a number of places to clean up code and make some public APIs easier to work with. This includes
* `atomic_replace_range` (not yet released, OK to change)
* `GetAllKeyVersions()` which gets a behavior change because of its unusual handling of empty keys.
* `DeleteFilesInRanges()`
* TODO in follow-up: `CompactRange()`
Most of the diff is associated updates and refactorings. Also
* Move some relevant things out of db.h to keep it as tidy as possible.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/13481
Test Plan: tests updated
Reviewed By: hx235
Differential Revision: D71747774
Pulled By: pdillinger
fbshipit-source-id: b4c8519608d119b8bceca9bb0fd778608f62a141
82794e0a4f