Add support for PubSub

[Dunedan]

This adds support for publishing game lists, rating lists and the leader board via PubSub. With matching support in pyrogenesis this will improve the performance and efficiency of the lobby, as the bots then don't have to manually send updates to each connected client, but the clients subscribe to the PubSub nodes and the XMPP server takes care of deliverying updates instead.

The previous behavior of sending stanzas with updates to every connected client is still available, but can be disabled with the --disable-legacy-lists command line flag. The performance benefits of PubSub do only materialize when the legacy behavior is disabled.