Update webrtc library to v3.0.0
This update required two main changes to how we use the library. First, we had to make sure we created the datachannel on the offering peer side before creating the offer. Second, we had to make sure we wait for the gathering of all candidates to complete since trickle-ice is enabled by default. See the release notes for more details: https://github.com/pion/webrtc/wiki/Release-WebRTC@v3.0.0.