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.

server/base/lib.rs

@@ -1,9 +1,10 @@
 // This file is part of Moonfire NVR, a security camera network video recorder.
-// Copyright (C) 2018 The Moonfire NVR Authors; see AUTHORS and LICENSE.txt.
+// Copyright (C) 2021 The Moonfire NVR Authors; see AUTHORS and LICENSE.txt.
 // SPDX-License-Identifier: GPL-v3.0-or-later WITH GPL-3.0-linking-exception.
 
 pub mod clock;
 mod error;
+pub mod shutdown;
 pub mod strutil;
 pub mod time;