ns_server should disable memcached console logging

By default memcached logs warnings and more serious errors to the console which in this case means stderr. By virtue of the normal stream redirection done by babysitter, this means that these log messages end up getting routed through babysitter to the babysitter log.

My understanding is that the reason we've arrived at this state of affairs is mostly an organic sequence of changes from earlier behavior. Memcached used to log everything through babysitter and leave log file rotation an compression to ns_server, but at some point decided to manage its own logs but leave the more serious logs in babysitter logs - kind of a belt and braces approach to logging. There have been cases in the past where these babysitter logs have proved useful, however, it's also the case that in the cases that memcached has tons of warnings, babysitter memory can spike massively making an already difficult situation worse. Given that memcached has taken control of managing its own log files, it seems that we should consider not enabling console logging.