[SPARK-4422][MLLIB]In some cases, Vectors.fromBreeze get wrong results.

cc mengxr

Author: GuoQiang Li <witgo@qq.com>

Closes #3281 from witgo/SPARK-4422 and squashes the following commits:

5f1fa5e [GuoQiang Li] import order
50783bd [GuoQiang Li] review commits
7a10123 [GuoQiang Li] In some cases, Vectors.fromBreeze get wrong results.
[SPARK-4410][SQL] Add support for external sort

Adds a new operator that uses Spark's `ExternalSort` class.  It is off by default now, but we might consider making it the default if benchmarks show that it does not regress performance.

Author: Michael Armbrust <michael@databricks.com>

Closes #3268 from marmbrus/externalSort and squashes the following commits:

48b9726 [Michael Armbrust] comments
b98799d [Michael Armbrust] Add test
afd7562 [Michael Armbrust] Add support for external sort.
unwrap for the ConstantObjectInspector
rebase the master