This is so far completely untested, for use by a new UI prototype.
It creates a new URL endpoint which sends one video/mp4 media segment
per key frame, with the dependent frames included. This means there will
be about one key frame interval of latency (typically about a second).
This seems hard to avoid, as mentioned in issue #59.
Fixes#46. If there are no video_sample_entries, it returns
InvalidArgument, which gets mapped to a HTTP 400. Various other failures
turn into non-500s as well.
There are many places that can & should be using typed errors, but it's
a start.
