https://github.com/pion/webrtc/pull/2696 introduced removing
datachannels from the sctptransport for better garbage collection.
That PR introduced a race condition for data channels created before
connection establishment. When an out of band negotiated data channel,
created before peerconnection establishment is detached, there's a race
between the data channel being removed from `r.dataChannels` and it
being copied in to the existing data channel slice in the
acceptDataChannels goroutine.
This PR fixes this race by copying the slice before any datachannels
could be detached.
This allows users of detached datachannels to garbage collect
resources associated with the datachannel and the sctp stream.
There is no functional change here.
Before we computed DataChannel IDs before signaling, this
is incorrect because IDs must take into account if we are
running an DTLS Client or Server.
This updates the DataChannel ID generation code to take this
into account before generating a streamId.
Resolves#908
