Cloning of lo_message objects

Sometimes its handy if you're able to clone a lo_message object, e.g. always then when you want to first aggregate received messages (e.g. from a bundle) and only later process them after you've already returned the method handler (as the message object is freed right after the method call by liblo).

The only way right now to clone a lo_message object is to first serialize it to a temporary buffer and then deserialize it again to a lo_message object which is utterly inefficient.

So here is a dedicated 'lo_message_clone' function which does it efficiently.