Copy Values (if necessary) in Datastore.Get()
Copy Values (if necessary) in Datastore.Get()

Summary:
Iterators provide ephemeral key/value pairs, but Get() should
return references that can be used long-term.

Test Plan: unit tests

Reviewers: tberman, jyeo

Reviewed By: tberman

CC: stephan, jimmy, justin, jyeo, tberman

Differential Revision: http://phabricator.thefactory.com/D982

thefactory/datastore,thefactory/datastore,thefactory/datastore,thefactory/datastore