feat: allow graceful shutdowns

Add a Shutdown(context.Context) method to the Poller. This will first
shutdown all active polling, preventing any new jobs from spawning. It
will then wait for either all jobs to finish, or for the context to
finish. If the context finishes, it will then force all jobs to end,
and then exit.

internal/app/cmd/daemon.go

@@ -122,9 +122,12 @@ func runDaemon(ctx context.Context, configFile *string) func(cmd *cobra.Command,
 
 		poller := poll.New(cfg, cli, runner)
 
-		poller.Poll(ctx)
+		go poller.Poll()
 
-		return nil
+		<-ctx.Done()
+		log.Infof("runner: %s gracefully shutting down", resp.Msg.Runner.Name)
+
+		return poller.Shutdown(context.Background())
 	}
 }