Change constraint on acq.provider. Instead of making code unique

Change constraint on acq.provider.  Instead of making code unique
by itself, make code + owner unique.

To change an existing table:

ALTER TABLE acq.provider
DROP CONSTRAINT provider_code_key;

ALTER TABLE acq.provider
ALTER COLUMN code SET NOT NULL;

ALTER TABLE acq.provider
ADD CONSTRAINT code_once_per_owner
	UNIQUE (code, owner);

git-svn-id: svn://svn.open-ils.org/ILS/trunk@13926 dcc99617-32d9-48b4-a31d-7c20da2025e4