[SPARK-14796][SQL] Add spark.sql.optimizer.inSetConversionThreshold config option.

## What changes were proposed in this pull request?

Currently, `OptimizeIn` optimizer replaces `In` expression into `InSet` expression if the size of set is greater than a constant, 10.
This issue aims to make a configuration `spark.sql.optimizer.inSetConversionThreshold` for that.

After this PR, `OptimizerIn` is configurable.
```scala
scala> sql("select a in (1,2,3) from (select explode(array(1,2)) a) T").explain()
== Physical Plan ==
WholeStageCodegen
:  +- Project [a#7 IN (1,2,3) AS (a IN (1, 2, 3))#8]
:     +- INPUT
+- Generate explode([1,2]), false, false, [a#7]
   +- Scan OneRowRelation[]

scala> sqlContext.setConf("spark.sql.optimizer.inSetConversionThreshold", "2")

scala> sql("select a in (1,2,3) from (select explode(array(1,2)) a) T").explain()
== Physical Plan ==
WholeStageCodegen
:  +- Project [a#16 INSET (1,2,3) AS (a IN (1, 2, 3))#17]
:     +- INPUT
+- Generate explode([1,2]), false, false, [a#16]
   +- Scan OneRowRelation[]
```

## How was this patch tested?

Pass the Jenkins tests (with a new testcase)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12562 from dongjoon-hyun/SPARK-14796.