Metadata from controller

[crepererum]

For certain read actions like getting the partition leader or getting a topic list we currently use an arbitrary broker. This is technically allowed and will eventually lead to the correct results (at least as far as my understanding goes). However this creates some delays, esp. when acting on a topic that was just created by the controller and our arbitrary broker is not the same.

We might instead use the controller connection in more places to reduce this delay.