Merge #3331

3331: Limit the number of concurrently opened indexes r=dureuill a=dureuill

# Pull Request

## Related issue
Relevant to #1841, fixes #3382

## What does this PR do?

### User standpoint

- Limit the number of concurrently opened indexes (currently, the number of indexes that can be concurrently opened is computed at startup)
- When too many an index is opened, the least recently used one is closed and its virtual memory released.
- This allows a user to have an arbitrary number of indexes of an arbitrary size

### Implementation standpoint

- Added a LRU cache map in `index-scheduler::lru`. A more complete implementation  (eg with helper functions not used here) is available but would better fit a dedicated crate.
- Use the LRU cache map in the `IndexScheduler`. To simplify the lifecycle of indexes, they are never removed from the cache when they are in the middle of a resize or delete operation. To achieve this, an intermediate `Vec` stores the UUIDs of the indexes that are in the middle of such an operation.
- Upon creating the index scheduler object, compute the total virtual memory that is adressable by using a dichotomic search on the max size of an index. Use this as a base to compute the number of indexes that can be open with 2TiB per index. If the virtual memory address space is lower than 2TiB, then only allow for 1 index of a fraction of that size.

Co-authored-by: Louis Dureuil <louis@meilisearch.com>

meilisearch/tests/common/server.rs

@@ -205,6 +205,7 @@ pub fn default_settings(dir: impl AsRef<Path>) -> Opt {
         indexer_options: IndexerOpts {
             // memory has to be unlimited because several meilisearch are running in test context.
             max_indexing_memory: MaxMemory::unlimited(),
+            skip_index_budget: true,
             ..Parser::parse_from(None as Option<&str>)
         },
         #[cfg(feature = "metrics")]