slack-19.0: backport vtorc optimisations, pt. 1

go/flags/endtoend/vtorc.txt

@@ -107,6 +107,7 @@ Flags:
       --topo_global_root string                                     the path of the global topology data in the global topology server
       --topo_global_server_address string                           the address of the global topology server
       --topo_implementation string                                  the topology implementation to use
+      --topo_read_concurrency int                                   Concurrency of topo reads. (default 32)
       --topo_zk_auth_file string                                    auth to use when connecting to the zk topo server, file contents should be <scheme>:<auth>, e.g., digest:user:pass
       --topo_zk_base_timeout duration                               zk base timeout (see zk.Connect) (default 30s)
       --topo_zk_max_concurrency int                                 maximum number of pending requests to send to a Zookeeper server. (default 64)