http://xmpp.org/extensions/xep-0060.html#impl-offline
one good solution given in the XEP is:
Specify a message type of "headline", which in most existing server
implementations will prevent offline storage.
but it seems the XEP does not provide a standard node configuration
item to define event and notification message type.
as a result, implmentation must choose between use of global
configuration (used for all hosted nodes), or use an undocumented node
option.
I think that would be great to have such documented note option.
something like pubsub#event_type and pubsub#notify_type
