mod_session_dbd: follow up to r1686122.

DBD entries should also have request lifetime.
Proposed by: Jacob Champion <jacob.champion ni.com>
Reviewed by: ylavic

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1687021 13f79535-47bb-0310-9956-ffa450edef68