fix(dump): Uncompress the dump IN the data.ms

When loading a dump with docker, we had two problems.
After creating a tempdirectory, uncompressing and re-indexing the dump:
1. We try to `move` the new “data.ms” onto the currently present
   one. The problem is that if the `data.ms` is a mount point because
   that's what peoples do with docker usually. We can't override
   a mount point, and thus we were throwing an error.
2. The tempdir is created in `/tmp`, which is usually quite small AND may not
   be on the same partition as the `data.ms`. This means when we tried to move
   the dump over the `data.ms`, it was also failing because we can't move data
   between two partitions.
==============
1 was fixed by deleting the *content* of the `data.ms` and moving the *content*
of the tempdir *inside* the `data.ms`. If someone tries to create volumes inside
the `data.ms` that's his problem, not ours.
2 was fixed by creating the tempdir *inside* of the `data.ms`. If a user mounted
its `data.ms` on a large partition, there is no reason he could not load a big
dump because his `/tmp` was too small. This solves the issue; now the dump is
extracted and indexed on the same partition the `data.ms` will lay.

fix #1833

meilisearch-lib/src/index_controller/dump_actor/mod.rs

@@ -185,7 +185,7 @@ pub fn load_dump(
     let mut meta_file = File::open(&meta_path)?;
     let meta: MetadataVersion = serde_json::from_reader(&mut meta_file)?;
 
-    let tmp_dst = tempfile::tempdir()?;
+    let tmp_dst = tempfile::tempdir_in(dst_path.as_ref())?;
 
     info!(
         "Loading dump {}, dump database version: {}, dump version: {}",
@@ -225,14 +225,34 @@ pub fn load_dump(
             indexer_opts,
         )?,
     }
-    // Persist and atomically rename the db
     let persisted_dump = tmp_dst.into_path();
+
+    // Delete everything in the `data.ms` except the tempdir.
     if dst_path.as_ref().exists() {
         warn!("Overwriting database at {}", dst_path.as_ref().display());
-        std::fs::remove_dir_all(&dst_path)?;
+        for file in dst_path.as_ref().read_dir().unwrap() {
+            let file = file.unwrap().path();
+            if file.file_name() == persisted_dump.file_name() {
+                continue;
+            }
+
+            if file.is_file() {
+                std::fs::remove_file(&file)?;
+            } else {
+                std::fs::remove_dir_all(&file)?;
+            }
+        }
     }
 
-    std::fs::rename(&persisted_dump, &dst_path)?;
+    // Move the whole content of the tempdir into the `data.ms`.
+    for file in persisted_dump.read_dir().unwrap() {
+        let file = file.unwrap().path();
+
+        std::fs::rename(&file, &dst_path.as_ref().join(file.file_name().unwrap()))?;
+    }
+
+    // Delete the empty tempdir.
+    std::fs::remove_dir_all(&persisted_dump)?;
 
     Ok(())
 }