Rather than running status-left, status-right and window title #() with popen

immediately every redraw, queue them up and run them in the background,
starting each once every status-interval. The actual status line uses the
output from the last run.

This brings several advantages:

- tmux itself may be called from inside #() without causing the server to hang;
- likewise, sleep or similar doesn't cause the server to block;
- commands aren't run excessively often when redrawing;
- commands shared by status-left and status-right, or used multiple times, will
  only be run once.

run-shell and if-shell still use system()/popen() but will be changed over to
use this too later.

Makefile

@@ -30,7 +30,7 @@ SRCS=	attributes.c buffer-poll.c buffer.c cfg.c client-fn.c \
 	cmd-up-pane.c cmd-display-message.c cmd-display-panes.c cmd.c \
 	colour.c environ.c grid-view.c grid.c input-keys.c \
 	imsg.c imsg-buffer.c input.c key-bindings.c key-string.c \
-	layout-set.c layout.c log.c \
+	layout-set.c layout.c log.c job.c \
 	mode-key.c names.c options-cmd.c options.c paste.c procname.c \
 	resize.c screen-redraw.c screen-write.c screen.c server-fn.c \
 	server-msg.c server.c session.c status.c tmux.c tty-keys.c tty-term.c \