Address the call number browsing performance problem raised in LP 690242

Address the call number browsing performance problem raised in LP 690242

The ORDER BY clause currently generated by call number browsing does not
have a sufficient index to use to assist the sorting of the returned rows,
and consequently does a sequential scan of the asset.call_number table.
Which, as you can imagine, is not fast for a system with more than a few
thousand call numbers.

This adds an index specifically to enable the query to go back to an
index scan instead of a sequential scan. We can investigate whether other
indexes should be removed to enable efficient data loading once we've
squashed the sequential scan problem.

git-svn-id: svn://svn.open-ils.org/ILS/branches/rel_2_0@19004 dcc99617-32d9-48b4-a31d-7c20da2025e4