Fix container date source with non-unique contents

Fix container date source with non-unique contents

It is possible for a bre container to have multiple items pointing
to the same bib. In these cases, a container_date sort attempt would
fail with:

ERROR:  more than one row returned by a subquery used as an
expression

This fix considers only a single date, the most recent time a given
bib was added to the container.

Signed-off-by: Jeff Godin <jgodin@tadl.org>