Fix divergence of stock serials schema and upgraded schema

Fix divergence of stock serials schema and upgraded schema

And additionally make a function used in a constraint on
serial.issuance.holding_code a little bit smarter.

These issues were reported to me by Galen Charlton.

The upgrade script included here will set serial.issuance.holding_code
to NULL in cases where the value doesn't validate, but in those cases
there is no part of Evergreen that could have made use of the data
anyway, and the data is just plain bad.

Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>
Signed-off-by: Mike Rylander <mrylander@gmail.com>