Avoid collection of RKMappingInfo when not using CoreData

Add a datasource method to indicate whether RKMappingInfo collection is necessary; it is only needed for CoreData presently, and is a performance hit otherwise.  Hotspot found while investigating #2065.

Timings before:
(Device) Mapping 5000 students with relationship mapping: 26.182683
(Simulator) Mapping 5000 students with relationship mapping: 3.718350

and after:
(Device) Mapping 5000 students with relationship mapping: 24.087797
(Simulator) Mapping 5000 students with relationship mapping: 3.415050
Please add documentation, including the intended behavior when the method is not implemented.