fix: ensure that pings don't get stuck behind writes

This:

1. Sets the max message size to 4 KiB. That should be large enough to
fill up most packets.
2. Sends pings/pongs first before processing messages.

This ensures that:

1. RTT times are accurate.
2. We don't end up thinking the connection is dead because a ping keeps
   getting stuck behind a message.

mux.go

@@ -75,7 +75,7 @@ func DefaultConfig() *Config {
 		MaxStreamWindowSize:     maxStreamWindow,
 		LogOutput:               os.Stderr,
 		ReadBufSize:             4096,
-		MaxMessageSize:          64 * 1024,
+		MaxMessageSize:          4 * 1024,
 		WriteCoalesceDelay:      100 * time.Microsecond,
 	}
 }