fix backups

* pluralize variable `backup_folder` -> `backups_folder`
* change env case `MEILI_backup_folder` -> `MEILI_BACKUPS_FOLDER`
* add miliseconds to backup ID to reduce colisions
* fix forgoten stats synchronization

meilisearch-http/tests/assets/dumps/v1/metadata.json

@@ -0,0 +1,12 @@
+{
+    "indices": [{
+            "uid": "test",
+            "primaryKey": "id"
+        }, {
+            "uid": "test2",
+            "primaryKey": "test2_id"
+        }
+    ],
+    "dbVersion": "0.13.0",
+    "dumpVersion": "1"
+}