Implement query iterators. (#74)

Query fetch has been refactored to work on top of query iterators.

Author: Chris Rossi

packages/google-cloud-ndb/MIGRATION_NOTES.md

@@ -156,6 +156,12 @@ The primary differences come from:
 - The `batch_size` and `prefetch_size` arguments to `Query.fetch` and
   `Query.fetch_async` are no longer supported. These were passed through
   directly to Datastore, which no longer supports these options.
+- The `index_list` method of `QueryIterator` is not implemented. Datastore no
+  longer returns this data with query results, so it is not available from the
+  API in this way. 
+- The `produce_cursors` query option is deprecated. Datastore always returns
+  cursors, where it can, and NDB always makes them available when possible.
+  This option can be passed in but it will be ignored.
 
 ## Privatization
 

packages/google-cloud-ndb/src/google/cloud/ndb/__init__.py

@@ -131,6 +131,8 @@
 from google.cloud.ndb.context import TransactionOptions
 from google.cloud.ndb._datastore_api import EVENTUAL
 from google.cloud.ndb._datastore_api import EVENTUAL_CONSISTENCY
+from google.cloud.ndb._datastore_query import Cursor
+from google.cloud.ndb._datastore_query import QueryIterator
 from google.cloud.ndb.key import Key
 from google.cloud.ndb.model import BlobKey
 from google.cloud.ndb.model import BlobKeyProperty
@@ -188,7 +190,6 @@
 from google.cloud.ndb.model import UserProperty
 from google.cloud.ndb.query import ConjunctionNode
 from google.cloud.ndb.query import AND
-from google.cloud.ndb.query import Cursor
 from google.cloud.ndb.query import DisjunctionNode
 from google.cloud.ndb.query import OR
 from google.cloud.ndb.query import FalseNode
@@ -201,7 +202,6 @@
 from google.cloud.ndb.query import ParameterNode
 from google.cloud.ndb.query import PostFilterNode
 from google.cloud.ndb.query import Query
-from google.cloud.ndb.query import QueryIterator
 from google.cloud.ndb.query import QueryOptions
 from google.cloud.ndb.query import RepeatedStructuredPropertyPredicate
 from google.cloud.ndb.tasklets import add_flow_exception