[SPARK-5976][MLLIB] Add partitioner to factors returned by ALS

The model trained by ALS requires partitioning information to do quick lookup of a user/item factor for making recommendation on individual requests. In the new implementation, we didn't set partitioners in the factors returned by ALS, which would cause performance regression.

srowen coderxiang

Author: Xiangrui Meng <meng@databricks.com>

Closes #4748 from mengxr/SPARK-5976 and squashes the following commits:

9373a09 [Xiangrui Meng] add partitioner to factors returned by ALS
260f183 [Xiangrui Meng] add a test for partitioner