Add support for multi-valued tables (#89728)
## What
Add support for multi-valued tables in `turbo-persistence`.
A multi-valued table allows multiple distinct values to be associated
with a single key. Each family is independently configured as
`SingleValue` (existing behavior) or `MultiValue` via the new
`FamilyKind` enum.
## Why
This will support the `TaskCache` table (implemented in #88904), where
keys will change to be _hashes_ instead of full `TaskType` values. This
greatly decreases DB size and speeds up queries due to smaller key
sizes, at the cost of hash collisions requiring multiple values per key.
## How
### API
- New `FamilyKind` enum (`SingleValue` / `MultiValue`) and per-family
`FamilyConfig` in `DbConfig`
- `get()` for single-valued families (panics if called on multi-valued)
- `get_multiple()` for multi-valued families, returns
`SmallVec<[ArcBytes; 1]>` — stack-allocated for the common 0–1 result
case, heap-scales when needed
- `put()` and `delete()` are unchanged — the family kind controls
dedup/compaction behavior
### Write path & compaction
- **Single-valued** (unchanged): last-write-wins per key
- **Multi-valued**: all values are maintained, deletions 'shadow' old
values.
- Deletion inserts a tombstone that shadows all older values for that
key across SST layers. Values written *after* the tombstone in the same
batch are retained.
- To avoid extra buffering logic the `MergeIter` semantics were changed
so it produces 'newest' entries first
* for SingleValues families this is no different since we only keep one
value
* for MultiValued families this makes dealing with tombstones trivial,
but does mean compaction will reverse the order of the set. For this
reason we make no guarantees about ordering.
### Read path
- Controlled by a `FIND_ALL` const generic on the internal lookup
methods
- **Single-valued** (`FIND_ALL=false`): binary search, return _last_
match, stop
* This fixes a bug where we might return Deleted when there is a value
in the SST depending on what the search algorithm found first
- **Multi-valued** (`FIND_ALL=true`): scan all matching entries in the
SST block, then continue to older SSTs. If a tombstone is found, stop
searching older layers.
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Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>
