[SPARK-13415][SQL] Visualize subquery in SQL web UI

## What changes were proposed in this pull request?

This PR support visualization for subquery in SQL web UI, also improve the explain of subquery, especially when it's used together with whole stage codegen.

For example:
```python
>>> sqlContext.range(100).registerTempTable("range")
>>> sqlContext.sql("select id / (select sum(id) from range) from range where id > (select id from range limit 1)").explain(True)
== Parsed Logical Plan ==
'Project [unresolvedalias(('id / subquery#9), None)]
:  +- 'SubqueryAlias subquery#9
:     +- 'Project [unresolvedalias('sum('id), None)]
:        +- 'UnresolvedRelation `range`, None
+- 'Filter ('id > subquery#8)
   :  +- 'SubqueryAlias subquery#8
   :     +- 'GlobalLimit 1
   :        +- 'LocalLimit 1
   :           +- 'Project [unresolvedalias('id, None)]
   :              +- 'UnresolvedRelation `range`, None
   +- 'UnresolvedRelation `range`, None

== Analyzed Logical Plan ==
(id / scalarsubquery()): double
Project [(cast(id#0L as double) / cast(subquery#9 as double)) AS (id / scalarsubquery())#11]
:  +- SubqueryAlias subquery#9
:     +- Aggregate [(sum(id#0L),mode=Complete,isDistinct=false) AS sum(id)#10L]
:        +- SubqueryAlias range
:           +- Range 0, 100, 1, 4, [id#0L]
+- Filter (id#0L > subquery#8)
   :  +- SubqueryAlias subquery#8
   :     +- GlobalLimit 1
   :        +- LocalLimit 1
   :           +- Project [id#0L]
   :              +- SubqueryAlias range
   :                 +- Range 0, 100, 1, 4, [id#0L]
   +- SubqueryAlias range
      +- Range 0, 100, 1, 4, [id#0L]

== Optimized Logical Plan ==
Project [(cast(id#0L as double) / cast(subquery#9 as double)) AS (id / scalarsubquery())#11]
:  +- SubqueryAlias subquery#9
:     +- Aggregate [(sum(id#0L),mode=Complete,isDistinct=false) AS sum(id)#10L]
:        +- Range 0, 100, 1, 4, [id#0L]
+- Filter (id#0L > subquery#8)
   :  +- SubqueryAlias subquery#8
   :     +- GlobalLimit 1
   :        +- LocalLimit 1
   :           +- Project [id#0L]
   :              +- Range 0, 100, 1, 4, [id#0L]
   +- Range 0, 100, 1, 4, [id#0L]

== Physical Plan ==
WholeStageCodegen
:  +- Project [(cast(id#0L as double) / cast(subquery#9 as double)) AS (id / scalarsubquery())#11]
:     :  +- Subquery subquery#9
:     :     +- WholeStageCodegen
:     :        :  +- TungstenAggregate(key=[], functions=[(sum(id#0L),mode=Final,isDistinct=false)], output=[sum(id)#10L])
:     :        :     +- INPUT
:     :        +- Exchange SinglePartition, None
:     :           +- WholeStageCodegen
:     :              :  +- TungstenAggregate(key=[], functions=[(sum(id#0L),mode=Partial,isDistinct=false)], output=[sum#14L])
:     :              :     +- Range 0, 1, 4, 100, [id#0L]
:     +- Filter (id#0L > subquery#8)
:        :  +- Subquery subquery#8
:        :     +- CollectLimit 1
:        :        +- WholeStageCodegen
:        :           :  +- Project [id#0L]
:        :           :     +- Range 0, 1, 4, 100, [id#0L]
:        +- Range 0, 1, 4, 100, [id#0L]
```

The web UI looks like:

![subquery](https://cloud.githubusercontent.com/assets/40902/13377963/932bcbae-dda7-11e5-82f7-03c9be85d77c.png)

This PR also change the tree structure of WholeStageCodegen to make it consistent than others. Before this change, Both WholeStageCodegen and InputAdapter hold a references to the same plans, those could be updated without notify another, causing problems, this is discovered by #11403 .

## How was this patch tested?

Existing tests, also manual tests with the example query, check the explain and web UI.

Author: Davies Liu <davies@databricks.com>

Closes #11417 from davies/viz_subquery.

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