bugfix worker DriverStateChanged state should match DriverState.FAILED

bugfix worker DriverStateChanged state should match DriverState.FAILED

Author: lianhuiwang <lianhuiwang09@gmail.com>

Closes #864 from lianhuiwang/master and squashes the following commits:

480ce94 [lianhuiwang] address aarondav comments
f2b5970 [lianhuiwang] bugfix worker DriverStateChanged state should match DriverState.FAILED
[SPARK-1926] [SQL] Nullability of Max/Min/First should be true.

Nullability of `Max`/`Min`/`First` should be `true` because they return `null` if there are no rows.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #881 from ueshin/issues/SPARK-1926 and squashes the following commits:

322610f [Takuya UESHIN] Fix nullability of Min/Max/First.
[SPARK-1915] [SQL] AverageFunction should not count if the evaluated value is null.

Average values are difference between the calculation is done partially or not partially.
Because `AverageFunction` (in not-partially calculation) counts even if the evaluated value is null.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #862 from ueshin/issues/SPARK-1915 and squashes the following commits:

b1ff3c0 [Takuya UESHIN] Modify AverageFunction not to count if the evaluated value is null.
[SQL] SPARK-1922

Allow underscore in column name of a struct field https://issues.apache.org/jira/browse/SPARK-1922 .

Author: LY Lai <ly.lai@vpon.com>

Closes #873 from lyuanlai/master and squashes the following commits:

2253263 [LY Lai] Allow underscore in struct field column name
[SPARK-1938] [SQL] ApproxCountDistinctMergeFunction should return Int value.

`ApproxCountDistinctMergeFunction` should return `Int` value because the `dataType` of `ApproxCountDistinct` is `IntegerType`.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #893 from ueshin/issues/SPARK-1938 and squashes the following commits:

3970e88 [Takuya UESHIN] Remove a superfluous line.
5ad7ec1 [Takuya UESHIN] Make dataType for each of CountDistinct, ApproxCountDistinctMerge and ApproxCountDistinct LongType.
cbe7c71 [Takuya UESHIN] Revert a change.
fc3ac0f [Takuya UESHIN] Fix evaluated value type of ApproxCountDistinctMergeFunction to Int.
Fix doc about NetworkWordCount/JavaNetworkWordCount usage of spark streaming

Usage: NetworkWordCount <master> <hostname> <port>
-->
Usage: NetworkWordCount <hostname> <port>

Usage: JavaNetworkWordCount <master> <hostname> <port>
-->
Usage: JavaNetworkWordCount <hostname> <port>

Author: jmu <jmujmu@gmail.com>

Closes #826 from jmu/master and squashes the following commits:

9fb7980 [jmu] Merge branch 'master' of https://github.com/jmu/spark
b9a6b02 [jmu] Fix doc for NetworkWordCount/JavaNetworkWordCount Usage: NetworkWordCount <master> <hostname> <port> --> Usage: NetworkWordCount <hostname> <port>