Avoid useless delay and master connections in wfWaitForSlaves() in runJobs.php

* Some jobs, like Parsoid ones, do not actually do master DB updates

Change-Id: I1a8830eafd7760d081df966bb423030dcdc96408
(cherry picked from commit 2979f0b6f1149803a190ff3127eccb6a5b6b106c)