Bug 22437: Delete older merge requests at new merge time

Bug 22437: Delete older merge requests at new merge time

If you merge A to B, we should remove older AA merges (regular
merge to itself) in the queue before deleting A.

Test plan:
[1] Set merge limit in prefs to say X.
[2] Find three authorities A, B, C with linkcount >X, >X, <X.
[3] Merge C to B. This creates a 'BB merge' in the queue.
    Auth C is deleted.
[4] Merge B to A. Creating AA and BA in the queue.
    Auth B is deleted.
    Optionally verify that BB is no longer in the queue.
[5] Run the merge_authorities.pl cron job.
    Verify that the linkcount to A is the former total of ABC.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Phil Ringnalda <philringnalda@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>