API inconsistency for StorageField and SortableField

[ctron]

In the Datastore API, when working with predicates the MessageField enum is used in combination with the StorableField interface.

However when it comes to sorting, the same API requires to use SortFieldImpl, which implements SortField but requests to use a String as field name.

IMHO this should be somewhat aligned to use a single Field interface which can use e.g. MessageField for sorting and storing.