[SQL] SPARK-1371 Hash Aggregation Improvements

Given:
```scala
case class Data(a: Int, b: Int)
val rdd =
  sparkContext
    .parallelize(1 to 200)
    .flatMap(_ => (1 to 50000).map(i => Data(i % 100, i)))
rdd.registerAsTable("data")
cacheTable("data")
```
Before:
```
SELECT COUNT(*) FROM data:[10000000]
16795.567ms
SELECT a, SUM(b) FROM data GROUP BY a
7536.436ms
SELECT SUM(b) FROM data
10954.1ms
```

After:
```
SELECT COUNT(*) FROM data:[10000000]
1372.175ms
SELECT a, SUM(b) FROM data GROUP BY a
2070.446ms
SELECT SUM(b) FROM data
958.969ms
```

Author: Michael Armbrust <michael@databricks.com>

Closes #295 from marmbrus/hashAgg and squashes the following commits:

ec63575 [Michael Armbrust] Add comment.
d0495a9 [Michael Armbrust] Use scaladoc instead.
b4a6887 [Michael Armbrust] Address review comments.
a2d90ba [Michael Armbrust] Capture child output statically to avoid issues with generators and serialization.
7c13112 [Michael Armbrust] Rewrite Aggregate operator to stream input and use projections.  Remove unused local RDD functions implicits.
5096f99 [Michael Armbrust] Make HiveUDAF fields transient since object inspectors are not serializable.
6a4b671 [Michael Armbrust] Add option to avoid binding operators expressions automatically.
92cca08 [Michael Armbrust] Always include serialization debug info when running tests.
1279df2 [Michael Armbrust] Increase default number of partitions.

1 files changed, 1 insertions, 0 deletions

diff --git a/project/SparkBuild.scala b/project/SparkBuild.scala
index d1e4b8b964..6b8740d9f2 100644
--- a/project/SparkBuild.scala
+++ b/project/SparkBuild.scala
@@ -178,6 +178,7 @@ object SparkBuild extends Build {
     fork := true,
     javaOptions in Test += "-Dspark.home=" + sparkHome,
     javaOptions in Test += "-Dspark.testing=1",
+    javaOptions in Test += "-Dsun.io.serialization.extendedDebugInfo=true",
     javaOptions in Test ++= System.getProperties.filter(_._1 startsWith "spark").map { case (k,v) => s"-D$k=$v" }.toSeq,
     javaOptions += "-Xmx3g",
     // Show full stack trace and duration in test cases.