[SPARK-1508][SQL] Add SQLConf to SQLContext.

This PR (1) introduces a new class SQLConf that stores key-value properties for a SQLContext (2) clean up the semantics of various forms of SET commands.

The SQLConf class unlocks user-controllable optimization opportunities; for example, user can now override the number of partitions used during an Exchange. A SQLConf can be accessed and modified programmatically through its getters and setters. It can also be modified through SET commands executed by `sql()` or `hql()`. Note that users now have the ability to change a particular property for different queries inside the same Spark job, unlike settings configured in SparkConf.

For SET commands: "SET" will return all properties currently set in a SQLConf, "SET key" will return the key-value pair (if set) or an undefined message, and "SET key=value" will call the setter on SQLConf, and if a HiveContext is used, it will be executed in Hive as well.

Author: Zongheng Yang <zongheng.y@gmail.com>

Closes #956 from concretevitamin/sqlconf and squashes the following commits:

4968c11 [Zongheng Yang] Very minor cleanup.
d74dde5 [Zongheng Yang] Remove the redundant mkQueryExecution() method.
c129b86 [Zongheng Yang] Merge remote-tracking branch 'upstream/master' into sqlconf
26c40eb [Zongheng Yang] Make SQLConf a trait and have SQLContext mix it in.
dd19666 [Zongheng Yang] Update a comment.
baa5d29 [Zongheng Yang] Remove default param for shuffle partitions accessor.
5f7e6d8 [Zongheng Yang] Add default num partitions.
22d9ed7 [Zongheng Yang] Fix output() of Set physical. Add SQLConf param accessor method.
e9856c4 [Zongheng Yang] Use java.util.Collections.synchronizedMap on a Java HashMap.
88dd0c8 [Zongheng Yang] Remove redundant SET Keyword.
271f0b1 [Zongheng Yang] Minor change.
f8983d1 [Zongheng Yang] Minor changes per review comments.
1ce8a5e [Zongheng Yang] Invoke runSqlHive() in SQLConf#get for the HiveContext case.
b766af9 [Zongheng Yang] Remove a test.
d52e1bd [Zongheng Yang] De-hardcode number of shuffle partitions for BasicOperators (read from SQLConf).
555599c [Zongheng Yang] Bullet-proof (relatively) parsing SET per review comment.
c2067e8 [Zongheng Yang] Mark SQLContext transient and put it in a second param list.
2ea8cdc [Zongheng Yang] Wrap long line.
41d7f09 [Zongheng Yang] Fix imports.
13279e6 [Zongheng Yang] Refactor the logic of eagerly processing SET commands.
b14b83e [Zongheng Yang] In a HiveContext, make SQLConf a subset of HiveConf.
6983180 [Zongheng Yang] Move a SET test to SQLQuerySuite and make it complete.
5b67985 [Zongheng Yang] New line at EOF.
c651797 [Zongheng Yang] Add commands.scala.
efd82db [Zongheng Yang] Clean up semantics of several cases of SET.
c1017c2 [Zongheng Yang] WIP in changing SetCommand to take two Options (for different semantics of SETs).
0f00d86 [Zongheng Yang] Add a test for singleton set command in SQL.
41acd75 [Zongheng Yang] Add a test for hql() in HiveQuerySuite.
2276929 [Zongheng Yang] Fix default hive result for set commands in HiveComparisonTest.
3b0c71b [Zongheng Yang] Remove Parser for set commands. A few other fixes.
d0c4578 [Zongheng Yang] Tmux typo.
0ecea46 [Zongheng Yang] Changes for HiveQl and HiveContext.
ce22d80 [Zongheng Yang] Fix parsing issues.
cb722c1 [Zongheng Yang] Finish up SQLConf patch.
4ebf362 [Zongheng Yang] First cut at SQLConf inside SQLContext.