Add support for storing SAML2 sessions

Store SAML2 session information in a table rather than with the
user entry so sessions can be persisted past IdP restarts and if
the user accesses the system via multiple browsers SLO will log
out all sessions, not just the user session that initiated the
logout.

https://fedorahosted.org/ipsilon/ticket/90

Signed-off-by: Rob Crittenden <rcritten@redhat.com>
Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>