shutdown better

After a frustrating search for a suitable channel to use for shutdown
(tokio::sync::watch::Receiver and
futures::future::Shared<tokio::sync::oneshot::Receiver> didn't look
quite right) in which I rethought my life decisions, I finally just made
my own (server/base/shutdown.rs). We can easily poll it or wait for it
in async or sync contexts. Most importantly, it's convenient; not that
it really matters here, but it's also efficient.

We now do a slightly better job of propagating a "graceful" shutdown
signal, and this channel will give us tools to improve it over time.

* Shut down even when writer or syncer operations are stuck. Fixes #117
* Not done yet: streamers should instantly shut down without waiting for
  a connection attempt or frame or something. I'll probably
  implement that when removing --rtsp-library=ffmpeg. The code should be
  cleaner then.
* Not done yet: fix a couple places that sleep for up to a second when
  they could shut down immediately. I just need to do the plumbing for
  mock clocks to work.

I also implemented an immediate shutdown mode, activated by a second
signal. I think this will mitigate the streamer wait situation.

guide/troubleshooting.md

@@ -272,13 +272,10 @@ clean up the excess files. Moonfire NVR will start working again immediately.
 
 If Moonfire NVR's own files are too large, follow this procedure:
 
-1.  Shut it down via `SIGKILL`:
+1.  Shut it down.
     ```console
-    $ sudo killall -KILL moonfire-nvr
+    $ sudo killall moonfire-nvr
     ```
-    (Be sure to use `-KILL`. It won't shut down properly on `SIGTERM` or `SIGINT`
-    when out of disk space due to [issue
-    #117](https://github.com/scottlamb/moonfire-nvr/issues/117).)
 2.  Reconfigure it use less disk space. See [Completing configuration through
     the UI](install.md#completing-configuration-through-the-ui) in the
     installation guide. Pay attention to the note about slack space.