fix: ensure asynchronous closing of database connection in delete_builds method

Author: DylanWelzel

biothings/hub/dataindex/mongo_build_cleanup.py

@@ -46,7 +46,7 @@ async def delete_builds(self, build_ids):
             result = await src_build.delete_many({"_id": {"$in": build_ids}})
             return {"deleted_count": result.deleted_count}
         finally:
-            conn.close()
+            await conn.close()
 
     def done(self, future):
         try: