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/src/cmds/check.rs

@@ -46,7 +46,7 @@ pub struct Args {
     trash_corrupt_rows: bool,
 }
 
-pub fn run(args: &Args) -> Result<i32, Error> {
+pub fn run(args: Args) -> Result<i32, Error> {
     let (_db_dir, mut conn) = super::open_conn(&args.db_dir, super::OpenMode::ReadWrite)?;
     check::run(
         &mut conn,