Trigger exactly one callback per early state change.

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.
Merge pull request #24 from dsandras/ds-dialog-cb-early-once

Trigger exactly one callback per early state change.
Credits go to Damien Sandras from Be IP s.a. @ http://www.beip.be