Major Now that KAFKA-2120 has been merged, we can improve a few things:
1. Remove `NetworkClient.completeAll`
2. Call `Selector.poll` with a timeout that is the minimum of timeout, metadata timeout and request timeout
3. Disallow `Selector.poll(-1)` as it's not used and not something that we want generally (and one can always use `Selector.poll(Integer.MAX_VALUE)` if one really wants it