[SPARK-4797] Replace breezeSquaredDistance

This PR replaces slow breezeSquaredDistance. A simple calculation involving 40000 squared distances between the vectors of 20000 dims shows:

* breezeSquaredDistance:  ~12 secs
* This PR: ~10.5 secs


v1 is sparse, so use faster access: ```v1.values(count)```
Also, it is common to use ```k``` instead of ```count```