Geo Cluster docs should discuss memcache

[clayg]

Somewhere in here:

http://docs.openstack.org/developer/swift/admin_guide.html#geographically-distributed-clusters

We should say that each regions proxies should use their own memcache pool to improve cache cohesiveness and response time.

When using something like tempauth there's a caveat trying to use a token generated in one region (and stored in it's local memcache pool) against another region (you'll have to re-auth for that region, and store the token per-region)

But with an authoritative auth server like Keystone or some LDAP integration each region would need only to validate the token once and cache the result in it's local memcache pool.

... but generally clients just stick to a single region when making requests - so it's sort of only win.