Implement an auto cleanup mechanism in the data layer
    
    This allows Store implementations to specify a TTL or expiration_time for
    entries in unique data tables.
    This should make it a lot simpler to implement cleaning up of things like
    session objects in a manner that is not tied to the actual data storage.
    
    Merges: #164
    Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>
    Reviewed-by: Ralph Bean <rbean@redhat.com>