perf(tcp): disable Nagle's algorithm on accepted connections

  Set TCP_NODELAY on all accepted TCP connections for both the Electrum
  RPC server and the REST HTTP server. Nagle's algorithm interacts poorly
  with TCP delayed ACKs on request/response protocols, adding up to
  200ms of latency per round trip on otherwise idle connections. Unix
  socket path in rest.rs is correctly left unchanged as TCP_NODELAY
  does not apply to Unix domain sockets.

Author: EddieHouston

src/electrum/server.rs

@@ -793,6 +793,7 @@ impl RPC {
                 stream
                     .set_nonblocking(false)
                     .expect("failed to set connection as blocking");
+                stream.set_nodelay(true).expect("failed to set TCP_NODELAY");
                 if acceptor.send(Some((stream, addr))).is_err() {
                     break; // receiver dropped, server is shutting down
                 }

src/rest.rs

@@ -601,7 +601,10 @@ async fn run_server(config: Arc<Config>, query: Arc<Query>, rx: oneshot::Receive
                 tokio::select! {
                     result = listener.accept() => {
                         match result {
-                            Ok((stream, _)) => spawn_conn(stream, Arc::clone(&query), Arc::clone(&config),  &graceful),
+                            Ok((stream, _)) => {
+                                stream.set_nodelay(true).expect("failed to set TCP_NODELAY");
+                                spawn_conn(stream, Arc::clone(&query), Arc::clone(&config),  &graceful);
+                            }
                             Err(e) => warn!("accept error: {}", e),
                         }
                     }