[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.
@pwendell requested this in the JIRA -- The main reason is that we'd like to make it easy to debug if users call `destroy` by mistake.