We were requesting two keyframes in a row.

Metadata OBUs can appear between the sequence header and the first
frame header.

RFC 6184 Section 8.5.1 implies that the sender will send a
a sequence parameter set in response to PLI.  Since a keyframe is
useless without parameters, use the SPS to detect keyframes.

Now that we start at 200kbps, we need to increase our rate faster.

Also allow selecting just a single connection.

This avoids issues with initialisation order.

We no longer send FIRs.

This is simpler and gets rid of ErrKeyframeNeeded.

This avoids a race condition since the delete messages are sent
asynchronously.

If simulcast is set to auto, disable simulcast when there are just
two users in the group.

This avoids code duplication in the various addUpMedia functions,
and makes it possible to replace a stream without reopening the
stream's source.  This will be required in order to change the
simulcast envelope.