[SPARK-16008][ML] Remove unnecessary serialization in logistic regression

JIRA: [SPARK-16008](https://issues.apache.org/jira/browse/SPARK-16008)

## What changes were proposed in this pull request?
`LogisticAggregator` stores references to two arrays of dimension `numFeatures` which are serialized before the combine op, unnecessarily. This results in the shuffle write being ~3x (for multiclass logistic regression, this number will go up) larger than it should be (in MLlib, for instance, it is 3x smaller).

This patch modifies `LogisticAggregator.add` to accept the two arrays as method parameters which avoids the serialization.

## How was this patch tested?

I tested this locally and verified the serialization reduction.

![image](https://cloud.githubusercontent.com/assets/7275795/16140387/d2974bac-3404-11e6-94f9-268860c931a2.png)

Additionally, I ran some tests of a 4 node cluster (4x48 cores, 4x128 GB RAM). Data set size of 2M rows and 10k features showed >2x iteration speedup.

Author: sethah <seth.hendrickson16@gmail.com>

Closes #13729 from sethah/lr_improvement.