upnp: auto-provisioning of upnp mappings

Implement a handler for automatic provisioning of port mappings through UPNP
and NAT-PMP protocols in NATed networks. A pool of UDP/TCP ports will be
allocated on start-up and periodically monitored such as when port allocation
is requested, it will be readily available. This will improve responsiveness
with some UPNP server implementations that may take several seconds to grant
a mapping request.

Gitlab: #335

Change-Id: I837945797d3b6bb337d3419bf77b4dc686558e5f