Cleanup on Connection, ConnectionManagerId, ConnectionManager classes part 2

Cleanup on Connection, ConnectionManagerId, and ConnectionManager classes part 2 while I was working at the code there to help IDE:
1. Remove unused imports
2. Remove parentheses in method calls that do not have side affect.
3. Add parentheses in method calls that do have side effect or not simple get to object properties.
4. Change if-else check (via isInstanceOf) for Connection class type with Scala expression for consistency and cleanliness.
5. Remove semicolon
6. Remove extra spaces.
7. Remove redundant return for consistency

Author: Henry Saputra <henry.saputra@gmail.com>

Closes #1157 from hsaputra/cleanup_connection_classes_part2 and squashes the following commits:

4be6906 [Henry Saputra] Fix Spark Scala style for line over 100 chars.
85b24f7 [Henry Saputra] Cleanup on Connection and ConnectionManager classes part 2 while I was working at the code there to help IDE: 1. Remove unused imports 2. Remove parentheses in method calls that do not have side affect. 3. Add parentheses in method calls that do have side effect. 4. Change if-else check (via isInstanceOf) for Connection class type with Scala expression for consitency and cleanliness. 5. Remove semicolon 6. Remove extra spaces.
[SPARK-2227] Support dfs command in SQL.

Note that nothing gets printed to the console because we don't properly maintain session state right now.

I will have a followup PR that fixes it.

Author: Reynold Xin <rxin@apache.org>

Closes #1167 from rxin/commands and squashes the following commits:

56f04f8 [Reynold Xin] [SPARK-2227] Support dfs command in SQL.
[SPARK-2124] Move aggregation into shuffle implementations

This PR is a sub-task of SPARK-2044 to move the execution of aggregation into shuffle implementations.

I leave `CoGoupedRDD` and `SubtractedRDD` unchanged because they have their implementations of aggregation. I'm not sure is it suitable to change these two RDDs.

Also I do not move sort related code of `OrderedRDDFunctions` into shuffle, this will be solved in another sub-task.

Author: jerryshao <saisai.shao@intel.com>

Closes #1064 from jerryshao/SPARK-2124 and squashes the following commits:

4a05a40 [jerryshao] Modify according to comments
1f7dcc8 [jerryshao] Style changes
50a2fd6 [jerryshao] Fix test suite issue after moving aggregator to Shuffle reader and writer
1a96190 [jerryshao] Code modification related to the ShuffledRDD
308f635 [jerryshao] initial works of move combiner to ShuffleManager's reader and writer