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/Cargo.toml

@@ -14,6 +14,7 @@ path = "lib.rs"
 
 [dependencies]
 failure = "0.1.1"
+futures = "0.3"
 lazy_static = "1.0"
 libc = "0.2"
 log = "0.4"
@@ -21,4 +22,5 @@ parking_lot = { version = "0.11.1", features = [] }
 nom = "7.0.0"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
+slab = "0.4"
 time = "0.1"