These wrappers allow a caller to provide UDPMux and TCPMux instances
to the ICE agent that represent multiple open ports. This can be
desirable in what would otherwise be single-port deployments, as it
increases the chance that one of the fixed ports will not be blocked
by a users firewall.
UDPMux before only worked with UDP4 traffic.
UDP6 traffic would simply be ignored.
This commit implements 2 connections per ufrag. When requesting a
connection for a ufrag the user must specify if they want IPv4 or IPv6.
Relates to pion/webrtc#1915
TCP/UDP Mux would have old entries of closed PacketConns from restarted
ICE Agents. Move cleanup code from Closing the ICE Agent into a shared
function and also call it during Restart.
