LP#1708291: add API for safe deleting various serial records

LP#1708291: add API for safe deleting various serial records

This adds routines for safely deleting subscriptions,
distributions and streams.

open-ils.serial.subscription.safe_delete
open-ils.serial.distribution.safe_delete
open-ils.serial.stream.safe_delete
open-ils.serial.subscription.safe_delete.dry_run
open-ils.serial.distribution.safe_delete.dry_run
open-ils.serial.stream.safe_delete.dry_run

Won't delete if there are attached serial.item records with a status other
than Expected, or if there are any attached non-deleted serial.unit
records.

The dry_run variants behave exactly the same except they don't actually
delete anything.

Signed-off-by: Jason Etheridge <jason@equinoxinitiative.org>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>