labs: restart slapd if it uses > 50% of memory
Add a cron that restarts slapd <s>once a week</s> to mitigate the slapd memory
leak issue, as suggested by Moritz.
Amended to restart it if it really uses more than 50% of the memory, check in an
hourly cron. As suggested by _joe_ in comments.
Copied from mediawiki::jobrunner where we do the same for hhvm.