Add shutdown grace period

[hannesdejager]

Improvement for #386

When doing a graceful shutdown you don't want things to take to long, so a grace period can be specified.

Logs for shutdown within grace period:

 Nov 12 16:07:19.160 INFO Shutting down FTP server
 Nov 12 16:07:19.160 INFO HTTP shutdown OK
lib: libunftp
 Nov 12 16:07:19.160 DEBG Shutting down within 10s
 Nov 12 16:07:19.161 DEBG Graceful shutdown complete
module: main

and the not so happy case:

 Nov 12 16:27:35.985 INFO HTTP shutdown OK
lib: libunftp
 Nov 12 16:27:35.985 DEBG Shutting down within 10s
 trace-id: 0x45bb0af218d17c28
  source: 127.0.0.1:57427
   Nov 12 16:27:35.985 INFO Shutting down control loop
 Nov 12 16:27:45.987 WARN Could not finish shutdown normally within grace period
module: main
 Nov 12 16:27:45.988 ERRO FTP server error: ServerError { msg: "shutdown error", source: ShutdownError { msg: "shutdown grace period expired" } }
 Nov 12 16:27:45.988 INFO FTP exiting

[hannesdejager]

I'm not really happy with the interface here, I may change it still.

current usage:

    let mut server = Server::with_authenticator(storage_backend, Arc::new(authenticator))
        .greeting("Welcome to unFTP")
        .passive_ports(start_port..end_port)
        .idle_session_timeout(idle_timeout)
        .logger(root_log.new(o!("lib" => "libunftp")))
        .passive_host(passive_host)
        .sitemd5(md5_setting)
        .shutdown_indicator(options::Shutdown::GracefulAcceptingConnections( async move {
            shutdown.recv().await.ok();
            info!(l, "Shutting down FTP server");
            Duration::from_secs(10)
        }))
        .metrics();