Stores the data to the app's sandbox, returning a unique identifier. 
      Syntax
         
         
            - 
            (
            NSString *
            ) 
            
               storeCache
            
            : 
            (
            id< SDMCaching >
            ) 
            cache_in
         
      
      Parameters
         
         
            - 
               cache_in – 
               cache object to be persisted
            
 
         
       
      Returns
         cache unique identifier The returned uid has to be managed by the caller in order to retrieve the persisted data