Add querying for instance with identifier

Added the ability to query existing instances of the model for an instance with the specified identifier so I don't have to use duplicate slug => league mappings

 + (instancetype)existingModelWithIdentifier: (id)identifier;
Knock yourself out ;^)

I wanted to get access to all instances of a model so I could do predicate querying on them, but FDCache doesn't have the ability to enumerate all objects, so if we're making FDCache thread-safe I'd like to add that at the same time.