Iterating over KafkaStreams.getAllMetadata() results in ConcurrentModificationException

`KafkaStreams.getAllMetadata()` returns `StreamsMetadataState.getAllMetadata()`. All the latter methods is `synchronized` it returns a reference to internal mutable state.  Not only does this break encapsulation, but it means any thread iterating over the returned collection when the metadata gets rebuilt will encounter a `ConcurrentModificationException`.