Added example with eventual read consistency in Node.js

[irri]

Documentation (https://cloud.google.com/datastore/docs/concepts/structuring_for_strong_consistency#datastore_read_policy) says:

// Read consistency cannot be specified in google-cloud-node.

However, this is implemented in the Node.js libraries as

query.run({ consistency: 'eventual' });

See googleapis/google-cloud-node#2599 (comment) and https://googlecloudplatform.github.io/google-cloud-node/#/docs/datastore/1.1.0/datastore/query?method=run

I've added example code to show how this is done in Node.js.

[codecov]

Codecov Report

Merging #480 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #480   +/-   ##
=======================================
  Coverage   97.16%   97.16%           
=======================================
  Files          13       13           
  Lines         458      458           
=======================================
  Hits          445      445           
  Misses         13       13

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[jmdobry]

Thanks!