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.
Interceptors are being accidentally misused by users.
The issue is that an Interceptor can be re-used between
multiple PeerConnections. Interceptors were designed to only be
single PeerConnection aware, so state is being corrupted.
Instead we are now going to provide InterceptorFactories. The default
API of pion/webrtc will now be safe to use between PeerConnections.
Resolves webrtc#1956
