feat(cancel): add NTATAG_CANCEL_3261
Add a new tag so that canceled transactions are not immediately marked as 'completed' (or worse: marked as 'terminated' in some cases). It is called '3261' as this new behavior is more compliant with RFC3261 (chap. 9.1, last paragraph). The transaction corresponding to the original request, and the CANCEL transaction can now complete independently.
Flexisip : BC/public/flexisip!1644
Closes FLEXISIP-739