k-means: Improve performance with array

Jira: MADLIB-454

It seems sparse vector was the bottleneck of the scalability.  We see 10
times faster in some of our use cases by using arrays instead of sparse
vectors.  Although we still have concerns around space efficiency, we
saw the arrays are also packed enough by the compression and toast mechanism.
Let's see by having this fix if our test cases verify the improvement.