This seems like a pretty expensive operation - what if there are hundreds of `BlockManagers`. It might make sense to say in the doc that this should only be used for testing. Otherwise people will come along and use it without understanding the performance implications.

Another thought here (let's talk offline) we should make it explicit which blocks the BlockManagerMaster is always informed about vs which ones it might not know about. Right now it's not made explicit anywhere and it's hard to reason about.