(Bug 43269) Hash titles in memcached keys

This change ensures that the target title is hashed to a fixed length, making
all memcached keys consistently 48 bytes ('shorturls:title:' + 32 byte MD5
hash). The change will also migrate values stored under the old key format.

Change-Id: I386794c46cbd8e203cbc45115a40d8f399e0939b