PDB-70 Add index on catalog_resources.exported

This increases performance for exported resource collection queries. For
postgresql the index is only a partial on exported = true, since indexing on
the very common 'false' case is not that effective.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #747 from kbarber/ticket/master/PDB-70-index-on-the-exported-column

PDB-70 Add index on catalog_resources.exported