Design and implement a way of including message type as part of every queue message.

We started with a "php-message-class" serialization thing, but I don't like it. Let's not use the SmartRecord pattern if we can avoid it. Instead, message type will be a (string const) enum that is interpreted at the receiving end.

Update the documentation.

Still use separate queues, cos that's just sane and convenient for attaching various hoses.