LP#902255: Protect against hold double-capture

LP#902255: Protect against hold double-capture

When the staff client is in Fast Scan (async) mode, it is possible
to double-scan at checkin and cause two holds to be captured by the
same item.  By creating a specially crafted unique index we ensure
that the database rejects this situation.

This protection would cause unhandled errors in the staff client, so
we also register a new string to be displayed when this particular
situation occurs, alerting staff to the possible double-scan of a
barcode at checkin IFF they are using async mode.

Signed-off-by: Mike Rylander <mrylander@gmail.com>