[SPARK-11111] [SQL] fast null-safe join

Currently, we use CartesianProduct for join with null-safe-equal condition.
```
scala> sqlContext.sql("select * from t a join t b on (a.i <=> b.i)").explain
== Physical Plan ==
TungstenProject [i#2,j#3,i#7,j#8]
 Filter (i#2 <=> i#7)
  CartesianProduct
   LocalTableScan [i#2,j#3], [[1,1]]
   LocalTableScan [i#7,j#8], [[1,1]]
```
Actually, we can have an equal-join condition as  `coalesce(i, default) = coalesce(b.i, default)`, then an partitioned join algorithm could be used.

After this PR, the plan will become:
```
>>> sqlContext.sql("select * from a join b ON a.id <=> b.id").explain()
TungstenProject [id#0L,id#1L]
 Filter (id#0L <=> id#1L)
  SortMergeJoin [coalesce(id#0L,0)], [coalesce(id#1L,0)]
   TungstenSort [coalesce(id#0L,0) ASC], false, 0
    TungstenExchange hashpartitioning(coalesce(id#0L,0),200)
     ConvertToUnsafe
      Scan PhysicalRDD[id#0L]
   TungstenSort [coalesce(id#1L,0) ASC], false, 0
    TungstenExchange hashpartitioning(coalesce(id#1L,0),200)
     ConvertToUnsafe
      Scan PhysicalRDD[id#1L]
```

Author: Davies Liu <davies@databricks.com>

Closes #9120 from davies/null_safe.