camera clock frequency correction

As described in design/time.md:

* get the realtime-monotonic once at the start of a run and use the
  monotonic clock afterward to avoid problems with local time steps

* on every recording, try to correct the latest local_time_delta at up
  to 500 ppm

Let's see how this works...

src/main.rs

@@ -181,7 +181,7 @@ fn run(args: Args, conn: rusqlite::Connection, signal: &chan::Receiver<chan_sign
         let env = streamer::Environment{
             db: &db,
             dir: &dir,
-            clock: &clock::REAL,
+            clocks: &clock::REAL,
             opener: &*stream::FFMPEG,
             shutdown: &shutdown,
         };