Add functionality to query executors for their local BlockStatuses

Not all blocks are reported to the master. In HttpBroadcast and
TorrentBroadcast, for instance, most blocks are not reported to master.
The lack of a mechanism to get local block statuses on each executor
makes it difficult to test the correctness of un/persisting a broadcast.

This new functionality, though only used for testing at the moment, is
general enough to be used for other things in the future.