[SPARK-4030] Make destroy public for broadcast variables

This change makes the destroy function public for broadcast variables. Motivation for the change is described in https://issues.apache.org/jira/browse/SPARK-4030.
This patch also logs where destroy was called from if a broadcast variable is used after destruction.
No - I actually prefer the synchronous version in my applications. I will add another destroy() which is always synchronous and make it public.