Add high-res timestamp to log file

While debugging our systems we noticed that the log files written by `memcached` are virtually useless as it's impossible to tell when a particular event has happened.

This is an add-on which depends on the Debian package [moreutils](https://packages.debian.org/wheezy/moreutils) - If not installed, nothing happens and the log file gets written as before, without timestamps.

I did some tests on a virtual machine and adding a timestamp slows writing to a log file considerably, but given that `memcached` is not overly verbose, we can live with it.