Trigger exactly one callback per early state change.

[dsandras]

There are several reasons behind this change:

1. Other callbacks are only triggered once for each state change.
2. SNOM phones send several 180 Ringing messages in a row. That triggers
   several early dialog callbacks, which in turn trigger several PUBLISH
   and NOTIFY through the pua_dialoginfo module. This can potentially lead
   to problems.

If accepted, please credit Damien Sandras from Be IP s.a. @ http://www.beip.be

[bogdan-iancu]

Indeed, it makes sense.