Optimize SQL query to use full text search (i.e. index) in order to find related documents (i.e. referencing documents) rather than like (which scan all table rows).

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@28548 20353a03-c40f-0410-a6d1-a30d3c3de9de