Drop the arn_value / arn_source columns on authority.record_entry and create an index...

Drop the arn_value / arn_source columns on authority.record_entry and create an index instead

The authority record number (ARN) was problematic when trying to generate
local authorities because the number had no correlation to the content of
the authority heading. By creating an index on authority records based on
their heading, thesaurus, and heading text, we can do a better job of
controlling the actual content of the authority records.

In a future release, we may convert this index to a unique index. 2.0
will give sites an opportunity to begin cleaning up their authorities
by identifying duplicate and problematic records through the
authority.normalize_heading(TEXT) database function.

git-svn-id: svn://svn.open-ils.org/ILS/branches/rel_2_0@17722 dcc99617-32d9-48b4-a31d-7c20da2025e4