This stops using parking_lot entirely. Since Rust 1.62, the std
implementations on Linux are direct futexes, not the boxed pthread
mutexes they used to be. No real reason to use parking_lot anymore, so
shed this dependency.
After a frustrating search for a suitable channel to use for shutdown
(tokio::sync:⌚: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.
