marionette: reduce importance of marionette communication

The log messages of what geckodriver sends and receives from the
Marionette server will now be logged at trace level.  This brings parity
to the way protocol chatter is logged in the Marionette server.

src/marionette.rs

@@ -1142,15 +1142,14 @@ impl MarionetteConnection {
         let poll_attempts = timeout / poll_interval;
         let mut poll_attempt = 0;
 
-        info!("Connecting to Marionette on {}:{}", DEFAULT_HOST, self.port);
         loop {
             match TcpStream::connect(&(DEFAULT_HOST, self.port)) {
                 Ok(stream) => {
                     self.stream = Some(stream);
                     break
                 },
                 Err(e) => {
-                    debug!("  connection attempt {}/{}", poll_attempt, poll_attempts);
+                    trace!("  connection attempt {}/{}", poll_attempt, poll_attempts);
                     if poll_attempt <= poll_attempts {
                         poll_attempt += 1;
                         sleep(Duration::from_millis(poll_interval));
@@ -1162,7 +1161,7 @@ impl MarionetteConnection {
             }
         };
 
-        debug!("TCP connection established");
+        debug!("Connected to Marionette on {}:{}", DEFAULT_HOST, self.port);
 
         try!(self.handshake());
         Ok(())
@@ -1217,7 +1216,7 @@ impl MarionetteConnection {
 
     fn send(&mut self, msg: Json) -> WebDriverResult<String> {
         let data = self.encode_msg(msg);
-        debug!("→ {}", data);
+        trace!("→ {}", data);
 
         match self.stream {
             Some(ref mut stream) => {
@@ -1291,7 +1290,7 @@ impl MarionetteConnection {
 
         // TODO(jgraham): Need to handle the error here
         let data = String::from_utf8(payload).unwrap();
-        debug!("← {}", data);
+        trace!("← {}", data);
 
         Ok(data)
     }