Makes UDPMux IPv4/IPv6 aware

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

udp_mux_universal_test.go

@@ -40,7 +40,7 @@ func TestUniversalUDPMux(t *testing.T) {
 }
 
 func testMuxSrflxConnection(t *testing.T, udpMux *UniversalUDPMuxDefault, ufrag string, network string) {
-	pktConn, err := udpMux.GetConn(ufrag)
+	pktConn, err := udpMux.GetConn(ufrag, false)
 	require.NoError(t, err, "error retrieving muxed connection for ufrag")
 	defer func() {
 		_ = pktConn.Close()