Bug 18070: Extend sub merge to remove fields for deleted authorities

Bug 18070: Extend sub merge to remove fields for deleted authorities

In order to accomplish this, we need to add some additional checks in
the merge routine. The actual change to remove the field, is quite
small.

Furthermore, we need to add a merge call in DelAuthority and adjust
the merge cron job accordingly.

The change is well supported by additional tests, including a simulation
of postponed removal via cron, if dontmerge is enabled.

Note: Deleting an authority with linked biblios is tested on the next
patch.

Test plan:
[1] Run t/db_dependent/Authorities/Merge.t
[2] Delete an authority without linked biblios from the Authorities
    module. If the indexer is not fast enough, wait a bit and refresh to
    verify that the authority is gone on authorities-home.pl.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>