The check prevented the creation of port forwardings which were assumed
to be present already. After this change the port forwardings which
might have been removed from the NAT device can be re-created.
In this commit, we introduce a new package `nat`. This package is
responsible for handling the different techniques for NAT traversals.
Specifically, we have implemented UPnP and NAT-PMP support. This will
allow users to easily advertise their nodes to the network as long as
their devices are behind a single NAT. Devices behind multiple NATs are
not supported.