[SPARK-4031] Make torrent broadcast read blocks on use.

This avoids reading broadcast variables when they are referenced in the closure but not used by the code.
Note: This is a WIP and a request for comments. I will update HttpBroadcast and add some tests if it sounds good.

cc @rxin @JoshRosen for review
Do we still need this method if it's just going to call the default readObject?