Improve performance of filtering custom fields by values used in exported projects on large databases

A customer with a Data Center instance and a very large database running on Oracle, suffers very long export times when filtering custom fields by values used in the exported projects. Finally it was confirmed that the problem was caused by the DB query that performs that filtering based on custom field values. Further tests confirmed that an alternative form of the query provided much better performance.

The fix version includes the improved queries, also for the "Used by" report.