[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
this is not your change, but we should log ms instead of s here. i will fix it later.