SPARK-2813:  [SQL] Implement SQRT() directly in Spark SQL

This PR adds a native implementation for SQL SQRT() and thus avoids delegating this function to Hive.
Hey @liancheng, I agree that it is possible to special-case this function (and can certainly do so for this PR).  I recently [wrote up what Hive does](http://chapeau.freevariable.com/2014/08/existing-system-coercion.html) and have some work in progress on a more general solution that should capture what Hive does.