Start using DTLS for loopback calls

[KaptenJansson]

Since SDES should not be used and we are hitting Chrome issues we should update AppRTC to use DTLS instead. This means we have to create two peerConnections since when using DTLS one peerConnection cannot act as a client and server.

I'm open for suggestions on how to solve this but I was thinking that we could create and manage the 2nd peerConnection in loopback.js. I guess it's also possible to manage it in call.js but there is already a lot going in there.

@juberti @samdutton @jiayliu WDYT?

[KaptenJansson]

This needs to revisited.

[fippo]

ping. SDES is going away...