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.

tmux.1

@@ -1437,6 +1437,11 @@ may contain any of the following special character sequences:
 The #(command) form executes
 .Ql command
 as a shell command and inserts the first line of its output.
+Note that shell commands are only executed once at the interval specified by
+the
+.Ic status-interval
+option: if the status line is redrawn in the meantime, the previous result is
+used.
 #[attributes] allows a comma-separated list of attributes to be specified,
 these may be
 .Ql fg=colour