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

server/Cargo.toml

@@ -26,11 +26,9 @@ blake3 = "1.0.0"
 bytes = "1"
 byteorder = "1.0"
 clap = { version = "2.33.3", default-features = false, features = ["color", "wrap_help"] }
-cstr = "0.2.5"
 cursive = "0.17.0"
 db = { package = "moonfire-db", path = "db" }
 failure = "0.1.1"
-ffmpeg = { package = "moonfire-ffmpeg", version = "0.0.2" }
 futures = "0.3"
 fnv = "1.0"
 h264-reader = "0.5.0"
@@ -66,6 +64,7 @@ url = "2.1.1"
 uuid = { version = "0.8", features = ["serde", "std", "v4"] }
 
 [dev-dependencies]
+mp4 = { git = "https://github.com/scottlamb/mp4-rust", branch = "moonfire" }
 num-rational = { version = "0.4.0", default-features = false, features = ["std"] }
 reqwest = { version = "0.11.0", default-features = false, features = ["json"] }
 tempfile = "3.2.0"