Signal handling

[rvolosatovs]

The server should listen on signals and gracefully shutdown and exit on appropriate signals.
Seems like we should be able to use https://docs.rs/signal-hook-async-std/latest/signal_hook_async_std/ for this.

Similarly to https://github.com/profianinc/drawbridge/blob/e84c7efcdafd97361b1fc0fc8acb26fa941733b8/tests/mod.rs#L37-L44 we should add a take_until to https://github.com/profianinc/drawbridge/blob/e84c7efcdafd97361b1fc0fc8acb26fa941733b8/src/main.rs#L69-L73, which should be passed a future, which is ready once a termination signal (probably SIGKILL and/or SIGTERM, please double-check what is applicable) is received.