This patch is an re-implementation of instant messaging system
for the SIP layers, to use endpoint's request than dialog's request.

This fix an issue where a packet drop during the request processing
causes the dialog to be shutdown with error code 408 afer 30 seconds.
This leds to shutdown of the whole attached invite session, so the
call itself it terminated.

Change-Id: I948c3eb1d6df992ed8364a01806cd92a42cf98a5
Tuleap: #1077
Tuleap: #1297