tonic: fix consistency issue with timeout func in transport/server builder

transport/server/mod.rs exposes a Builder for the Server, and most of the
Builder functions return Self (a change made for issue #115), but timeout
returns mut& Self. This change makes timeout consistent with the rest of
the Builder api by returning Self.

Fixes: #900
Refs: #115

Author: jdahlq

tonic/src/transport/server/mod.rs

@@ -186,12 +186,14 @@ impl<L> Server<L> {
     /// # use tonic::transport::Server;
     /// # use tower_service::Service;
     /// # use std::time::Duration;
-    /// # let mut builder = Server::builder();
+    /// # let builder = Server::builder();
     /// builder.timeout(Duration::from_secs(30));
     /// ```
-    pub fn timeout(&mut self, timeout: Duration) -> &mut Self {
-        self.timeout = Some(timeout);
-        self
+    pub fn timeout(self, timeout: Duration) -> Self {
+        Server {
+            timeout: Some(timeout),
+            ..self
+        }
     }
 
     /// Sets the [`SETTINGS_INITIAL_WINDOW_SIZE`][spec] option for HTTP2