drop ffmpeg support

* switch the config interface over to use Retina and make the test
  button honor rtsp_transport = udp.

* adjust the threading model of the Retina streaming code.

  Before, it spawned a background future that read from the runtime and
  wrote to a channel. Other calls read from this channel.

  After, it does work directly from within the block_on calls (no
  channels).

  The immediate motivation was that the config interface didn't have
  another runtime handy. And passing in a current thread runtime
  deadlocked. I later learned this is a difference between
  Runtime::block_on and Handle::block_on. The former will drive IO and
  timers; the latter will not.

  But this is also more efficient to avoid so many thread hand-offs.
  Both the context switches and the extra spinning that
  tokio appears to do as mentioned here:
  https://github.com/scottlamb/retina/issues/5#issuecomment-871971550

  This may not be the final word on the threading model. Eventually
  I may not have per-stream writing threads at all. But I think it will
  be easier to look at this after getting rid of the separate
  `moonfire-nvr config` subcommand in favor of a web interface.

* in tests, read `.mp4` files via the `mp4` crate rather than ffmpeg.
  The annoying part is that this doesn't parse edit lists; oh well.

* simplify the `Opener` interface. Formerly, it'd take either a RTSP
  URL or a path to a `.mp4` file, and they'd share some code because
  they both sometimes used ffmpeg. Now, they're totally different
  libraries (`retina` vs `mp4`). Pull the latter out to a `testutil`
  module with a different interface that exposes more of the `mp4`
  stuff. Now `Opener` is just for RTSP.

* simplify the h264 module. It had a lot of logic to deal with Annex B.
  Retina doesn't use this encoding.

Fixes #36
Fixes #126

docker/deploy.bash

@@ -19,7 +19,6 @@ uname -a
 export DEBIAN_FRONTEND=noninteractive
 time apt-get update
 time apt-get install --assume-yes --no-install-recommends \
-    ffmpeg \
     libncurses6 \
     libncursesw6 \
     locales \

docker/dev.bash

@@ -78,10 +78,6 @@ time apt-get update
 
 # Install the packages for the target architecture.
 packages+=(
-    ffmpeg"${apt_target_suffix}"
-    libavcodec-dev"${apt_target_suffix}"
-    libavformat-dev"${apt_target_suffix}"
-    libavutil-dev"${apt_target_suffix}"
     libncurses-dev"${apt_target_suffix}"
     libsqlite3-dev"${apt_target_suffix}"
 )