Added SignalLogger to HistoryServer.

This was omitted in #1260. @aarondav

Author: Reynold Xin <rxin@apache.org>

Closes #1300 from rxin/historyServer and squashes the following commits:

af720a3 [Reynold Xin] Added SignalLogger to HistoryServer.
[SPARK-2370][SQL] Decrease metadata retrieved for partitioned hive queries.

Author: Michael Armbrust <michael@databricks.com>

Closes #1305 from marmbrus/usePrunerPartitions and squashes the following commits:

744aa20 [Michael Armbrust] Use getAllPartitionsForPruner instead of getPartitions, which avoids retrieving auth data
[SPARK-2306]:BoundedPriorityQueue is private and not registered with Kry...

Due to the non registration of BoundedPriorityQueue  with kryoserializer, operations which are dependend on BoundedPriorityQueue are giving exceptions.One such instance is using top along with kryo serialization.
Fixed the issue by registering BoundedPriorityQueue with kryoserializer.

Author: ankit.bhardwaj <ankit.bhardwaj@guavus.com>

Closes #1299 from AnkitBhardwaj12/BoundedPriorityQueueWithKryoIssue and squashes the following commits:

a4ae8ed [ankit.bhardwaj] [SPARK-2306]:BoundedPriorityQueue is private and not registered with Kryo
[SPARK-2366] [SQL] Add column pruning for the right side of LeftSemi join.

The right side of `LeftSemi` join needs columns only used in join condition.

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

Closes #1301 from ueshin/issues/SPARK-2366 and squashes the following commits:

7677a39 [Takuya UESHIN] Update comments.
786d3a0 [Takuya UESHIN] Rename method name.
e0957b1 [Takuya UESHIN] Add column pruning for the right side of LeftSemi join.
[SPARK-2327] [SQL] Fix nullabilities of Join/Generate/Aggregate.

Fix nullabilities of `Join`/`Generate`/`Aggregate` because:
- Output attributes of opposite side of `OuterJoin` should be nullable.
- Output attributes of generater side of `Generate` should be nullable if `join` is `true` and `outer` is `true`.
- `AttributeReference` of `computedAggregates` of `Aggregate` should be the same as `aggregateExpression`'s.

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

Closes #1266 from ueshin/issues/SPARK-2327 and squashes the following commits:

3ace83a [Takuya UESHIN] Add withNullability to Attribute and use it to change nullabilities.
df1ae53 [Takuya UESHIN] Modify nullabilize to leave attribute if not resolved.
799ce56 [Takuya UESHIN] Add nullabilization to Generate of SparkPlan.
a0fc9bc [Takuya UESHIN] Fix scalastyle errors.
0e31e37 [Takuya UESHIN] Fix Aggregate resultAttribute nullabilities.
09532ec [Takuya UESHIN] Fix Generate output nullabilities.
f20f196 [Takuya UESHIN] Fix Join output nullabilities.
Started to implement the simplifiable decision tree
Deleted old decision tree. Renamed top node to root node
Renamed the tree to original name
WIP - start implementing tree simplifier
Add license