Bug 37000: (Bug 36120 follow-up) Improve reliability of database update

Bug 37000: (Bug 36120 follow-up) Improve reliability of database update

This patch adds a series of fallthroughs to ensure pickup_library_id is
always set prior to adding the NOT NULL constraint.

We initially only looked at items.homebranch but as that's a nullable
field itself, we now look at items.holdingbranch before finally
defaulting to the first available branch in the branches table in the
worst case.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>