SPARK-2680: Lower spark.shuffle.memoryFraction to 0.2 by default

Author: Matei Zaharia <matei@databricks.com>

Closes #1593 from mateiz/spark-2680 and squashes the following commits:

3c949c4 [Matei Zaharia] Lower spark.shuffle.memoryFraction to 0.2 by default
[SPARK-2361][MLLIB] Use broadcast instead of serializing data directly into task closure

We saw task serialization problems with large feature dimension, which could be avoid if we don't serialize data directly into task but use broadcast variables. This PR uses broadcast in both training and prediction and adds tests to make sure the task size is small.

Author: Xiangrui Meng <meng@databricks.com>

Closes #1427 from mengxr/broadcast-new and squashes the following commits:

b9a1228 [Xiangrui Meng] style update
b97c184 [Xiangrui Meng] minimal change to LBFGS
9ebadcc [Xiangrui Meng] add task size test to RowMatrix
9427bf0 [Xiangrui Meng] add task size tests to linear methods
e0a5cf2 [Xiangrui Meng] add task size test to GD
28a8411 [Xiangrui Meng] add test for NaiveBayes
380778c [Xiangrui Meng] update KMeans test
bccab92 [Xiangrui Meng] add task size test to LBFGS
02103ba [Xiangrui Meng] remove print
e73d68e [Xiangrui Meng] update tests for k-means
174cb15 [Xiangrui Meng] use local-cluster for test with a small akka.frameSize
1928a5a [Xiangrui Meng] add test for KMeans task size
e00c2da [Xiangrui Meng] use broadcast in GD, KMeans
010d076 [Xiangrui Meng] modify NaiveBayesModel and GLM to use broadcast