* Split `SegmentEntry`
Add `ImmutableSegmentEntry` for operations that can be applied to
immutable segments, and make the `SegmentEntry` as it subtrait.
* Rename to `NonAppendableSegmentEntry`
It differs semantically from `ImmutableSegmentEntry` by allowing point
deletion. Move point deletion to the trait too.
* Fix docstring
* use NonAppendableSegmentEntry where possible
---------
Co-authored-by: Andrey Vasnetsov <andrey@vasnetsov.com>
* WIP: make proxy segment non-appendable
* make deleted points and index changes private to Proxy Segment
* consistently remove empty unused segment from holder on unproxifying and the end of optimization
* remove/fix obsolete test
* Fix comment, some general adjustments
* Also don't remove non-original segments
* Use ahash for map of proxy index changes
* Inline format arguments
* Simplify has point branching
* Assert that proxy flushes to latest wrapped segment version
* make vector deletetion a CoW operation
* relax debug assertions
* Update lib/shard/src/proxy_segment/mod.rs
* fix test_partial_snapshot_empty
---------
Co-authored-by: timvisee <tim@visee.me>
Co-authored-by: Tim Visée <tim+github@visee.me>
