[SPARK-4570][SQL]add BroadcastLeftSemiJoinHash

JIRA issue: [SPARK-4570](https://issues.apache.org/jira/browse/SPARK-4570)
We are planning to create a `BroadcastLeftSemiJoinHash` to implement the broadcast join for `left semijoin`
In left semijoin :
If the size of data from right side is smaller than the user-settable threshold `AUTO_BROADCASTJOIN_THRESHOLD`, 
the planner would mark it as the `broadcast` relation and mark the other relation as the stream side. The broadcast table will be broadcasted to all of the executors involved in the join, as a `org.apache.spark.broadcast.Broadcast` object. It will use `joins.BroadcastLeftSemiJoinHash`.,else it will use `joins.LeftSemiJoinHash`.

The benchmark suggests these  made the optimized version 4x faster  when `left semijoin` 
<pre><code>
Original:
left semi join : 9288 ms 
Optimized:
left semi join : 1963 ms 
</code></pre>
The micro benchmark load `data1/kv3.txt` into a normal Hive table.
Benchmark code:
<pre><code>
 def benchmark(f: => Unit) = {
    val begin = System.currentTimeMillis()
    f
    val end = System.currentTimeMillis()
    end - begin
  }
  val sc = new SparkContext(
    new SparkConf()
      .setMaster("local")
      .setAppName(getClass.getSimpleName.stripSuffix("$")))
  val hiveContext = new HiveContext(sc)
  import hiveContext._
  sql("drop table if exists left_table")
  sql("drop table if exists right_table")
  sql( """create table left_table (key int, value string)
       """.stripMargin)
  sql( s"""load data local inpath "/data1/kv3.txt" into table left_table""")
  sql( """create table right_table (key int, value string)
       """.stripMargin)
  sql(
    """
      |from left_table
      |insert overwrite table right_table
      |select left_table.key, left_table.value
    """.stripMargin)

  val leftSimeJoin = sql(
    """select a.key from left_table a
      |left semi join right_table b on a.key = b.key""".stripMargin)
  val leftSemiJoinDuration = benchmark(leftSimeJoin.count())
  println(s"left semi join : $leftSemiJoinDuration ms ")
</code></pre>
Why do we need to up the threshold?  Can we just write the tests against relations that we have statistics on?