[SPARK-5419][Mllib] Fix the logic in Vectors.sqdist

The current implementation in Vectors.sqdist is not efficient because of allocating temp arrays. There is also a bug in the code `v1.indices.length / v1.size < 0.5`. This pr fixes the bug and refactors sqdist without allocating new arrays.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #4217 from viirya/fix_sqdist and squashes the following commits:

e8b0b3d [Liang-Chi Hsieh] For review comments.
314c424 [Liang-Chi Hsieh] Fix sqdist bug.