Removing cursor and more results from query object.

This is a follow up to a discussion in #423 and #425.

Also
- Adding `fetch_page()` method to return the needed properties
  no longer available on the `Query` object.
- Made `fetch_page()` fail if `more_results` is `NOT_FINISHED`, which
  seems to be intended for batched requests.
- Added `start_cursor` and `end_cursor` properties to Query.
- Updated docstrings to reflect change
- Updated tests to ensure the returned cursor was always set.
- Streamlined `fetch()`/`fetch_page()` tests to run via a single
  tweakable helper.
- Updated a `datastore.connection` test to construct and return an
  initialized `QueryResultBatch` to simulate a valid cursor.
- Updated `regression.clear_datastore` and `regression.datastore` to
  reflect changed API surface.
@pcostell As I see it we have two options:

- Support repeated kinds but mention on docstring that it will fail
- Only support single kind and update when the feature is implemented (I'm unclear on how that would be announced)

Which do you prefer?