Model modifier for root query and -to-many relations only

[IlyaSemenov]

Currently, a model/table modifier runs for all queries against that model:

• root query
• -to-many relations
• -to-one relations

In many cases, such as when using the modifier for sorting or limiting data access, running it for -to-one relations is useless.

I propose to add new r.model({ manyModifier }) (not sure about the naming) that will only run for root query and -to-many relations.