fix: do not set read_consistency for queries

[sorced-jim]

In Cloud Datastore ancestor queries are intended to be strong by default
(https://cloud.google.com/datastore/docs/concepts/structuring_for_strong_consistency) .

In Cloud Firestore in Datastore mode, all queries should be strongly
consistent by default (https://cloud.google.com/datastore/docs/upgrade-to-firestore).

closes #666

[chrisrossi]

Hi @sorced-jim ,

Can you help me understand the problem being solved here? If you wanted to open an issue explaining a bug in NDB and marking this PR as fixing that issue, that would be helpful.

Thank you!

[sorced-jim]

Chris, the problem being solved is that NDB is incorrectly setting the default query consistency as noted from the documented links. In general Cloud Datastore or Cloud Firestore libraries should avoid duplicating service default (what is attempted currently with the code).
n
I'll go back to my examples, Cloud Datastore, & ndb users should expect that ancestor queries are strongly consistent, with the current version of Cloud NDB this is not true because the default read consistency is set instead of doing what other client libraries do and let the service select the default consistency.

For Cloud Firestore in Datastore mode, all queries should be strongly consistency until the user explicitly requests an eventually consistent request. With the current version of Cloud NDB this is not true because the default read consistency is set instead of doing what other client libraries do and let the service select the default.

[craiglabenz]

LGTM