Memcached StorageService

[Cantor, Scott]

(And the link to the CAS document helped a lot, in that it explains that
you get HA because of how this particular client library works, which has
not been clear to me in past discussions ;-)

We should obviously be able to do a similar analysis of the issues for
SAML artifacts and the IdP's session cache. And replay I guess, but replay
is a relatively small consideration for the IdP.

-- Scott