Add memkeys to role::memcached

memkeys is a top-like utility for memcache keys by Tumblr, ported to C++
from a Ruby utility by Etsy. It's useful to debug memcache regressions.

Try it on memcached servers, for now, although eventually it could be
also very useful in appservers themselves as they would have the full
view of all keys.

Change-Id: Ida4b76470871bc1911d878fea44fc71bda58e004