`ORMRepositoryPDO` fetching creates massive queries due to duplicated relation selections

ORMRepositoryPDO::getDomainFetchQuery(...) iterates relations and for each one calls ORMHelper::getRelationsSelection(...) , thus adding the selection clause for all the relations on each iteration – so we end up with the relation selection clause in the query X times, where X is the number of relations defined for the domain. This is valid SQL but is an obvious performance issue.

We should call ORMHelper::getRelationSelection(...) passing $rel.