datastore.Entity should not know about the dataset

[dhermes]

This is redundant and potentially problematic.

The Key will know the dataset and that is sufficient.

See #445 and #336 for some context.

[dhermes]

Ditto for transaction, since it should be implied from the Querys and Keys involved in the transaction.

[dhermes]

More in https://docs.google.com/document/d/1xKw8Tz6lfN5uOcv0my4uUgFSeDoRzwCLho2UGlBpqDU/edit

[dhermes]

Closing out, this will be covered by #477.