SIP Witch multipath call termination

When exosip2 normally terminates a call, it does not support a status message in the body. In the case of a multi-pathed call that is picked up by one destination, and hence the call is cancelled to the remaining destinations, those destinations show a "missed call". By having call status as part of termination, we can notify the terminated extensions that the call was picked up elsewhere. This requires changes to both sipwitch and eXosip2; the latter proposed exosip2 api changes have already been discussed with and approved by Aymeric Moizard.