[STREAMING] SPARK-2343: Fix QueueInputDStream with oneAtATime false

Fix QueueInputDStream which was not removing dequeued items when used with the oneAtATime flag disabled.

Author: Manuel Laflamme <manuel.laflamme@gmail.com>

Closes #1285 from mlaflamm/spark-2343 and squashes the following commits:

61c9e38 [Manuel Laflamme] Unit tests for queue input stream
c51d029 [Manuel Laflamme] Fix QueueInputDStream with oneAtATime false

(cherry picked from commit 0eb11527d13083ced215e3fda44ed849198a57cb)
Signed-off-by: Tathagata Das <tathagata.das1565@gmail.com>
[SPARK-2417][MLlib] Fix DecisionTree tests

Fixes test failures introduced by https://github.com/apache/spark/pull/1316.

For both the regression and classification cases,
val stats is the InformationGainStats for the best tree split.
stats.predict is the predicted value for the data, before the split is made.
Since 600 of the 1,000 values generated by DecisionTreeSuite.generateCategoricalDataPoints() are 1.0 and the rest 0.0, the regression tree and classification tree both correctly predict a value of 0.6 for this data now, and the assertions have been changed to reflect that.

Author: johnnywalleye <jsondag@gmail.com>

Closes #1343 from johnnywalleye/decision-tree-tests and squashes the following commits:

ef80603 [johnnywalleye] [SPARK-2417][MLlib] Fix DecisionTree tests

(cherry picked from commit d35e3db2325931492b64890125a70579bc3b587b)
Signed-off-by: Xiangrui Meng <meng@databricks.com>