cli: ensure ordering of rpc server messages (#183558)

* cli: ensure ordering of rpc server messages

Sending lots of messages to a stream would block them around the async
tokio mutex, which is "fair" so doesn't preserve ordering. Instead, use
the write_loop approach I introduced to the server_multiplexer for the
same reason some time ago.

* fix clippy

cli/src/util/sync.rs

@@ -4,11 +4,9 @@
  *--------------------------------------------------------------------------------------------*/
 use async_trait::async_trait;
 use std::{marker::PhantomData, sync::Arc};
-use tokio::{
-	sync::{
-		broadcast, mpsc,
-		watch::{self, error::RecvError},
-	},
+use tokio::sync::{
+	broadcast, mpsc,
+	watch::{self, error::RecvError},
 };
 
 #[derive(Clone)]