[rql rewrite] until a better solution is found raise BadSchemaDefinition when two inlined relations with security on an optional variable is used, explaining how to bypass it

[rql rewriting] handle case where we've and optional inlined relation in the original query. Also, we should append EXISTS even in subquery to avoid inserting duplicates in results

[rql rewriting] handle case where we want to insert snippet on a variable that has previously been moved to a subquery