[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 <matei@databricks.com>

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

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