Memcached proxy compatibility

We currently use W3TC but are looking to move to WP-FFPC instead.  It didn't work out-of-the box because we're using Twitter's Nutcracker, which only supports the ASCII protocol and immediately disconnects when asked for stats.  This PR allows the protocol type to be selected, and attempts a set instead of issuing the stat command.  Nutcracker is "down" without these changes.