In broker, convert 'subscribe', 'unsubscribe' and 'publish' events to middlewares

[jondubois]

This will make it possible to implement guaranteed message delivery. E.g. https://github.com/SocketCluster/scc-broker-client/issues/3

This should be done in sc-broker module https://github.com/SocketCluster/sc-broker/blob/master/scbroker.js#L400