5124: Optimize Prefixes and Merges r=ManyTheFish a=Kerollmops
In this PR, we plan to optimize the read of LMDB to use read the entries in lexicographic order and better use the memory-mapping OS cache:
- Optimize the prefix generation for word position docids (`@manythefish)`
- Optimize the parallel merging of the caches to sort entries before merging the caches (`@kerollmops)`
## Benchmarks on 1cpu 2gb gpo3 (5k IOps)
Before on the tag meilisearch-v1.12.0-rc.3.
```
word_position_docids:merge_and_send_docids: 988s
compute_word_fst: 23.3s
word_pair_proximity_docids:merge_and_send_docids: 428s
compute_word_prefix_fid_docids:recompute_modified_prefixes: 76.3s
compute_word_prefix_position_docids:recompute_modified_prefixes:from_prefixes: 429s
```
After sorting the whole `HashMap`s in a `Vec` on this branch.
```
word_position_docids:merge_and_send_docids: 202s
compute_word_fst: 20.4s
word_pair_proximity_docids:merge_and_send_docids: 427s
compute_word_prefix_fid_docids:recompute_modified_prefixes: 65.5s
compute_word_prefix_position_docids:recompute_modified_prefixes:from_prefixes: 62.5s
```
Co-authored-by: ManyTheFish <many@meilisearch.com>
Co-authored-by: Kerollmops <clement@meilisearch.com>
