[SPARK-5714][Mllib] Refactor initial step of LDA to remove redundant operations

The `initialState` of LDA performs several RDD operations that looks redundant. This pr tries to simplify these operations.

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

Closes #4501 from viirya/sim_lda and squashes the following commits:

4870fe4 [Liang-Chi Hsieh] For comments.
9af1487 [Liang-Chi Hsieh] Refactor initial step of LDA to remove redundant operations.