A token with no "sub" field is now treated just like one with an
empty "sub".  In addition, all times are treated with a slack of 5s.

Also use Duration.Abs.

We used to have unbounded channels embedded within rtpconn
and webClient.  Make the structure explicit and testable.

This may happen if we're running over plain HTTP.
Thanks to kovmir.

gorilla/websocket is not updated, since 1.5.1 doesn't look good
to me.

We used to accept the browser's default, which varied widely
between devices.

This reverts commit 69445d67.

The onclose callback might want to access c.sc.

Replaced the non-existent `track` property in `RTCStatsType`
with the valid `inbound-rtp` property.

Fixes a crash in the TURN server.

This avoids restarting the stream when changing simulcast
settings.

Active TCP candidates were added in Pion 3.2.11.

Simulcast interacts strangely with VP9.