UPNP/NAT-PMP - add a handler for provisioned port mapping
Add a handler for the provisioned port mappings (see #830 (moved)). The handler must automatically maintain a pool of mapping ready for use whenever a call is started (outgoing/incoming). The handler must create new mappings if the number of available mappings drops under a pre-defined limit, and must also delete unecessary mappings when the number of mapping exceeds a defined max limit.