Expose consistent client method set about sarama HOT 2 CLOSED

While poking around various implementations of #18 I finally figured out what I want to do with disconnectBroker (ie how to get rid of it). It really has two purposes:

• it forces a metadata refresh to happen on the next client request for that broker (ie call to Leader or similar)
• even if the metadata isn't stale, it bounces the broker connection so that (for example) a stale connection to the correct broker is put back in a good state

The metadata refresh we can already force with a call to one of the public Refresh methods. The connection bounce is interesting, because I originally added this comment to the client.update method: "[if the broker hasn't changed] ignore it, replacing our existing one would just bounce the connection". However if we make the metadata refresh safely bounce connections for any broker that isn't stale we could in principle replace all "public" calls to disconnectBroker with straight calls to RefreshTopicMetadata.

We still have one internal call to it that would need dealing with (perhaps it just gets folded into the any method?) and parts of it are still tangled up in #15 but I think I've made progress in at least understanding the problem space.

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All of the methods I started this ticket for have been created (a while ago, in most cases). The only open question is broker disconnection, which is still better-tracked by #15. This can be closed.

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