[SPARK-3084] [SQL] Collect broadcasted tables in parallel in joins

BroadcastHashJoin has a broadcastFuture variable that tries to collect
the broadcasted table in a separate thread, but this doesn't help
because it's a lazy val that only gets initialized when you attempt to
build the RDD. Thus queries that broadcast multiple tables would collect
and broadcast them sequentially. I changed this to a val to let it start
collecting right when the operator is created.

Author: Matei Zaharia <[email protected]>

Closes apache#1990 from mateiz/spark-3084 and squashes the following commits:

f468766 [Matei Zaharia] [SPARK-3084] Collect broadcasted tables in parallel in joins

(cherry picked from commit 6a13dca)
Signed-off-by: Michael Armbrust <[email protected]>

Author: mateiz

sql/core/src/main/scala/org/apache/spark/sql/execution/joins.scala

@@ -424,7 +424,7 @@ case class BroadcastHashJoin(
     UnspecifiedDistribution :: UnspecifiedDistribution :: Nil
 
   @transient
-  lazy val broadcastFuture = future {
+  val broadcastFuture = future {
     sparkContext.broadcast(buildPlan.executeCollect())
   }