Use plugin-specific configuration, better expiration

Use plugin-specific configuration, better expiration

Use a SAML2 plugin specific option to specify the database uri
for sessions.

Use a much more robust method to find sessions that need
expiration (thanks Patrick).

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

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