Slave lag check tweaks to JobRunner

* Do not block forever, but wait up to 10 seconds. Likewise,
  check the lag times in memcached on startup. This at least
  lets runners avoid lagged wikis but still work on others.
* Made a few small related documentation and code cleanups.

Change-Id: Ic1339bab54cba6b6cbea7d97a80ff87c7c5c87af