[SPARK-2094][SQL] Exactly once command

## Related JIRA issues

- Main issue:

  - [SPARK-2094](https://issues.apache.org/jira/browse/SPARK-2094): Ensure exactly once semantics for DDL/Commands

- Issues resolved as dependencies:

  - [SPARK-2081](https://issues.apache.org/jira/browse/SPARK-2081): Undefine output() from the abstract class Command and implement it in concrete subclasses
  - [SPARK-2128](https://issues.apache.org/jira/browse/SPARK-2128): No plan for DESCRIBE
  - [SPARK-1852](https://issues.apache.org/jira/browse/SPARK-1852): SparkSQL Queries with Sorts run before the user asks them to

- Other related issue:

  - [SPARK-2129](https://issues.apache.org/jira/browse/SPARK-2129): NPE thrown while lookup a view

    Two test cases, `join_view` and `mergejoin_mixed`, within the `HiveCompatibilitySuite` are removed from the whitelist to workaround this issue.

## PR Overview

This PR defines physical plans for DDL statements and commands and wrap their side effects in a lazy field `PhysicalCommand.sideEffectResult`, so that they are executed eagerly and exactly once.  Also, as a positive side effect, now DDL statements and commands can be turned into proper `SchemaRDD`s and let user query the execution results.

This PR defines schemas for the following DDL/commands:

- EXPLAIN command

  - `plan`: String, the plan explanation
  
- SET command

  - `key`: String, the key(s) of the propert(y/ies) being set or queried
  - `value`: String, the value(s) of the propert(y/ies) being queried

- Other Hive naive command

  - `result`: String, execution result returned by Hive
  
  **NOTE**: We should refine schemas for different native commands by defining physical plans for them in the future.

## Examples

### EXPLAIN command

Take the "EXPLAIN" command as an example, we first execute the command and obtain a `SchemaRDD` at the same time, then query the `plan` field with the schema DSL:

```
scala> loadTestTable("src")
...

scala> val q0 = hql("EXPLAIN SELECT key, COUNT(*) FROM src GROUP BY key")
...
q0: org.apache.spark.sql.SchemaRDD =
SchemaRDD[0] at RDD at SchemaRDD.scala:98
== Query Plan ==
ExplainCommandPhysical [plan#11:0]
 Aggregate false, [key#4], [key#4,SUM(PartialCount#6L) AS c_1#2L]
  Exchange (HashPartitioning [key#4:0], 200)
   Exchange (HashPartitioning [key#4:0], 200)
    Aggregate true, [key#4], [key#4,COUNT(1) AS PartialCount#6L]
     HiveTableScan [key#4], (MetastoreRelation default, src, None), None

scala> q0.select('plan).collect()
...
[ExplainCommandPhysical [plan#24:0]
 Aggregate false, [key#17], [key#17,SUM(PartialCount#19L) AS c_1#2L]
  Exchange (HashPartitioning [key#17:0], 200)
   Exchange (HashPartitioning [key#17:0], 200)
    Aggregate true, [key#17], [key#17,COUNT(1) AS PartialCount#19L]
     HiveTableScan [key#17], (MetastoreRelation default, src, None), None]

scala>
```

### SET command

In this example we query all the properties set in `SQLConf`, register the result as a table, and then query the table with HiveQL: 

```
scala> val q1 = hql("SET")
...
q1: org.apache.spark.sql.SchemaRDD =
SchemaRDD[7] at RDD at SchemaRDD.scala:98
== Query Plan ==
<SET command: executed by Hive, and noted by SQLContext>

scala> q1.registerAsTable("properties")

scala> hql("SELECT key, value FROM properties ORDER BY key LIMIT 10").foreach(println)
...
== Query Plan ==
TakeOrdered 10, [key#51:0 ASC]
 Project [key#51:0,value#52:1]
  SetCommandPhysical None, None, [key#55:0,value#56:1]), which has no missing parents
14/06/12 12:19:27 INFO scheduler.DAGScheduler: Submitting 1 missing tasks from Stage 5 (SchemaRDD[21] at RDD at SchemaRDD.scala:98
== Query Plan ==
TakeOrdered 10, [key#51:0 ASC]
 Project [key#51:0,value#52:1]
  SetCommandPhysical None, None, [key#55:0,value#56:1])
...
[datanucleus.autoCreateSchema,true]
[datanucleus.autoStartMechanismMode,checked]
[datanucleus.cache.level2,false]
[datanucleus.cache.level2.type,none]
[datanucleus.connectionPoolingType,BONECP]
[datanucleus.fixedDatastore,false]
[datanucleus.identifierFactory,datanucleus1]
[datanucleus.plugin.pluginRegistryBundleCheck,LOG]
[datanucleus.rdbms.useLegacyNativeValueStrategy,true]
[datanucleus.storeManagerType,rdbms]

scala>
```

### "Exactly once" semantics

At last, an example of the "exactly once" semantics:

```
scala> val q2 = hql("CREATE TABLE t1(key INT, value STRING)")
...
q2: org.apache.spark.sql.SchemaRDD =
SchemaRDD[28] at RDD at SchemaRDD.scala:98
== Query Plan ==
<Native command: executed by Hive>

scala> table("t1")
...
res9: org.apache.spark.sql.SchemaRDD =
SchemaRDD[32] at RDD at SchemaRDD.scala:98
== Query Plan ==
HiveTableScan [key#58,value#59], (MetastoreRelation default, t1, None), None

scala> q2.collect()
...
res10: Array[org.apache.spark.sql.Row] = Array([])

scala>
```

As we can see, the "CREATE TABLE" command is executed eagerly right after the `SchemaRDD` is created, and referencing the `SchemaRDD` again won't trigger a duplicated execution.
Makes sense. And a little off topic, why is `SQLContext.emptyResult` an `RDD[Row]` with a single empty row instead of simply an empty `RDD[Row]`?