Add simple in-memory cache from nova.

Nova's simple in-memory cache replicates the memcache interface,
so clients can cache things in memcache or in memory using
the same commands. Using memcached requires having the client
library installed and the memcached_servers config option set.
Callers can also pass in a list of memcached servers when they
initialize the client.

Change-Id: I831142a36797b04006cba4792df803e09f6fd69b
Merge "Add simple in-memory cache from nova."