mod_server_presence: fire events when peers are added or removed ...
- Fire "peer-subscription-completed", when the approval handshake is done
- Fire "peer-subscription-removed", when a peer subscription is lifted.
mod_pubsub: split standard handlers and make 'em exportable.
pubsub_aux.lib: add missing set_service export.
pubsub_aux.lib: add missing constants.
mod_pubsub: remove unused constant.