[fti] add cw_fti_index_rql_limit method
Improves on and deprecate cw_fti_index_rql_queries: execute the rql
directly, so that we don't need to compute the set of eids up-front, but
can just keep track of the last seen entity. Use the new method in
reindex_entities.
Keep calling cw_fti_index_rql_queries if it has been redefined (e.g in
cubes).
Related to #3621392.
