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.

CHANGELOG.md

@@ -23,6 +23,13 @@ Each release is tagged in Git and on the Docker repository
     reliability with old live555 versions when using TCP also.
 *   improve compatibility with cameras that send non-compliant SDP, including
     models from Geovision and Anpviz.
+*   fix [#117](https://github.com/scottlamb/moonfire-nvr/issues/117): honor
+*   shut down requests when out of disk space, instead of retrying forever.
+*   shut down immediately on a second `SIGINT` or `SIGTERM`. The normal
+    "graceful" shutdown will still be slow in some cases, eg when waiting for a
+    RTSP UDP session to time out after a `TEARDOWN` failure. This allows the
+    impatient to get fast results with ctrl-C when running interactively, rather
+    than having to use `SIGKILL` from another terminal.
 
 ## `v0.6.5` (2021-08-13)