add on_close to abstract and simplify ws_client socket closing

backend/common/src/ws_client/mod.rs

@@ -20,6 +20,8 @@ mod connect;
 mod receiver;
 /// The channel based send interface
 mod sender;
+/// A close helper that we use in sender/receiver.
+mod on_close;
 
 pub use connect::{connect, ConnectError, Connection, RawReceiver, RawSender};
 pub use receiver::{Receiver, RecvError, RecvMessage};