The first time we drop, we may already have mapped a number of
packets with the identity mapping.  Insert an identity mapping
in Drop.

Also extend any existing mapping when inserting out-of-order mappings.

We were not rewriting in this case.

Maintain spatial layer information, and drop lower layers when
possible.  Yields a 20% saving with VP9.

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.