[rql2sql] closes #1892473: enhance handling of optional inlined relation
* some outer join aren't supported while they could
* consider newly available 'optcomparisons' st information on variables (rql 0.30)
[schema] use definition for `use_email` relation instead of type declaration (closes: #1894929)
This will be more adaptable for other existent `use_relastion` as the one in the cube Person.