(bug 44067) add --max-time param to dispatchChanges

dispatchChanges.php should support a max-time parameter that determines how
long the dispatcher runs before exiting. By setting this to a value that is
equivalent to the interval at which dispatchChanges.php instances are launched
(e.g. via cron), we can keep a constant number of instances running
persistently.

Change-Id: I8e151f62cd9619e137fb9d515c4e73949e77e0f1