datastore: simplify Iterator
datastore: simplify Iterator

The Iterator now only stores the current results and cursor, as well as
the minimal amount of information issue future API calls.

Related to issue #268

Change-Id: Iecf2378044922761294a6105a744d50fe8fbb401
Reviewed-on: https://code-review.googlesource.com/4930
Reviewed-by: Michael McGreevy <e70fec140fa6773c6cfd45cb9b436bd6334f4c24@golang.org>

googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go