Bug 11445: avoid sending duplicate hold waiting notifications

Bug 11445: avoid sending duplicate hold waiting notifications

This patch fixes a problem where a patron could receive duplicate
hold waiting notifications.  For example, this could happen if a
circ operator checked in an item more than once and confirmed the
same hold each time.

To test:

[1] Set up a test patron that received hold waiting notifications.
[2] Put an item on hold for the patron, then check the item in
    and confirm the hold.  Verify that a hold notification is
    sent (or inspect the message_queue table).
[3] Check the item in again and confirm the hold again.  A duplicate
    hold notification will be generated.
[4] Apply the patch.
[5] Repeat steps 2 and 3.  This time, only one notification should be
    generated.
[6] Verify that prove -v t/db_dependent/Reserves.t passes.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Passes all tests and QA script.
Works as described.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit fe08a0ef97ad2fbab72b857d61bec364fd93d9e9)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>