Watch ZK for broker updates and react accordingly

@krasserm 

This PR attempts to address https://github.com/krasserm/akka-persistence-kafka/issues/14. It keeps the same start up behaviour as before, i.e, read the /broker/ids directory synchronously.

1. Watch the /brokers/ids directory (and children) for changes
2. When changes occur, update the journal actor configuration and respawn the writer actors
3. When changes occur, update the snapshot actor configuration