Update our dependency on pion/sdp from v2 to v3, to match pion/webrtc
v3. This requires some changes in how we parse out addresses from ice
candidates. This will ease tor browser builds of snowflake since we are
now only relying on one version of pion/sdp instead of two different
ones.
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.
