Add client_mut function to Consumer

[sevagh]

Lets us get a mut client. This helped me when doing reset_metadata(). The doc string might need to change, whatever you like.

[dead10ck]

This looks good to me, thanks! I'll get it merged when I get a chance.

[dead10ck]

@sevagh published on crates.io as v0.6.2. Thanks again!

[xitep]

sorry for the long pause from my side; what exactly is the use-case you are after? in general i believe this change is not "safe". it allows the following (which doesn't really make sense to me):

1. c = Consumer::with_topic("topic-a")...
2. c.poll(); ...
3. { let mut cm = c.client_mut(); cm.reset_metadata(); cm.load_metadata("topic-b"); }
4. c.poll();

The latter (and any following) call to .poll will result in no message - it'll not fail. but the consumer will continue (needlessly) maintaining offsets for "topic-a".

maybe you're use-case/required behaviour can be implemented directly in the consumer (i guess it's about reloading partition re-assignments for the consumed topics) or be exposed directly in the consumer's API. generally, i do not favor client code working with the consumer (or producer) and KafkaClient at the same time. they represent different levels of abstraction that we don't want to couple tightly.

[sevagh]

This is the code:

fn ...(&mut self) -> {
    loop {
            let msgsets = match self.kafka_consumer.poll() {
                Ok(result) => result,
                Err(kafka::error::Error(kafka::error::ErrorKind::Kafka(KafkaCode::NotLeaderForPartition), _)) => {
                    let mut client = self.kafka_consumer.client_mut();
                    client.load_metadata_all()?;
                    continue
                }
                Err(ref e) => {
                    error!("kafka poll error: {}", e);
                    continue;
                }
            };

There's no real way to do this without client_mut(), or without re-creating the consumer in the loop if the topic leader changes.

[xitep]

@sevagh many thanks for the example! i think this is exactly what #81 is meant to address. until then we can of course keep the client_mut method.