Add support for prefetching fields
This makes it possible to specify the fields that should be
prefetched as part of a getForFields query. Doing a prefetch on a
ReferenceProperty when it is known that the property will be
referenced in (almost all) entities is far more efficient than
doing the invididual fetches sequentially.
A list of bugs is available in the Google App Engine SDK project on Google Code.
The issue tracker is at http://code.google.com/p/googleappengine/issues/.