This change adapts pion/ice to use a new interface for most network
related operations. The interface was formerly a simple struct vnet.Net
which was originally intended to facilicate testing. By replacing it
with an interface we have greater flexibility and allow users to hook
into the networking stack by providing their own implementation of
the interface.
It isn't possible to send media if a MediaEngine has no codecs. This
catches a common misconfiguration issues that users find themselves in.
Resolves#1702
