MS-266 Improve topk reduce time by using multi-threads

Former-commit-id: f19c6d78f7f9ab619de792fbb43efd86da0b07a1

cpp/conf/server_config.template

@@ -8,7 +8,7 @@ db_config:
   db_path: @MILVUS_DB_PATH@             # milvus data storage path
   db_slave_path:                        # secondry data storage path, split by semicolon
 
-  parallel_reduce: true                 # use multi-threads to reduce topk result
+  parallel_reduce: false                 # use multi-threads to reduce topk result
 
   # URI format: dialect://username:password@host:port/database
   # All parts except dialect are optional, but you MUST include the delimiters