Added a split cache

Author: fulmicoton

.gitignore

@@ -6,6 +6,7 @@
 **/flamegraph.svg
 local/**
 quickwit/quickwit-ui/package-lock.json
+**/.DS_Store
 
 
 # Remove Cargo.lock from gitignore if creating an executable, leave it for libraries

docs/internals/searcher-split-cache.md

@@ -0,0 +1,24 @@
+
+# Searcher split cache
+
+Quickwit includes a split cache. It can be useful for specific workloads:
+- to improve performance
+- to reduce the cost associated with GET requests.
+
+The split cache stores entire split files on disk.
+It works under the following configurable constraints:
+- number of concurrent download
+- amount of disk space
+- number of on-disk files.
+
+Searcher get tipped by indexers about the existence of splits (for which they have the best affinity).
+They also might learn about split existence, upon read requests.
+
+The searcher is then in charge of maintaining an in-memory datastructure with a bounded list of splits it knows about and their score.
+The current strategy for admission/evicton is a simple LRU logic.
+
+If the most recently accessed splits not already in cache has been accessed, we consider downloading it.
+If the limits have been reached, we only proceed to eviction if one of the split currently
+in cache has been less recently accessed.
+
+