TODO:

    define entity-map interface/protocol
    two implementations
        eager - EntityMap contains Entity
	lazy  - EntityMap contains [keychain valmap], only creates Entity objects when needed

dsmap dependencies

    DatastoreMap:
	emap??
	    keychain-to-key

entity-map dependencies:

    EntityMap:
	  to-keychain
	  keychain-to-key
	  props-to-map
	  get-val-clj
	  get-val-ds
