C<delete_expired_sessions> method is there so that regularly scheduled
maintenance scripts can give your backend the opportunity to clean up.
+=head2 Early Finalization
+
+By default the main session plugin will finalize during body finalization
+which ensures that all controller code related to the session has completed.
+However some storage plugins may wish to finalize earlier, during header
+finalization. For example a storage that saved state in a client cookie
+would wish this. If a storage plugin wants to finalize early it should set
+$c->_needs_early_session_finalization to true. Please note that if you
+do this in a storage plugin, you should warn users not to attempt to change
+or add session keys if you use a streaming or socket interface such as
+$c->res->write, $c->res->write_fh or $c->req->io_fh.
+
=cut