[SPARK-6065] [MLlib] Optimize word2vec.findSynonyms using blas calls

1. Use blas calls to find the dot product between two vectors.
2. Prevent re-computing the L2 norm of the given vector for each word in model.

Author: MechCoder <manojkumarsivaraj334@gmail.com>

Closes #5467 from MechCoder/spark-6065 and squashes the following commits:

dd0b0b2 [MechCoder] Preallocate wordVectors
ffc9240 [MechCoder] Minor
6b74c81 [MechCoder] Switch back to native blas calls
da1642d [MechCoder] Explicit types and indexing
64575b0 [MechCoder] Save indexedmap and a wordvecmat instead of matrix
fbe0108 [MechCoder] Made the following changes 1. Calculate norms during initialization. 2. Use Blas calls from linalg.blas
1350cf3 [MechCoder] [SPARK-6065] Optimize word2vec.findSynonynms using blas calls