[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.
Okay if that's the case I'd propose making `destroy()` public and keeping `destroy(blocking: Boolean)` as private. That way we minimize the surface area of public APIs.