[WIP] KAFKA-4677: Avoid unnecessary task movement across threads of the same process during rebalance

[dguy]

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[dguy]

@guozhangwang - i sent you a private message relating to this. Anyway, i thought i'd put what i've done this far here so you can have a look. You might have some other/better ideas on how we should go about it.
Currently if we had a scenario like so:
5 consumers, 3 active tasks, 3 standby tasks.
3 consumers each have 1 previous active task and 1 previous standby task.
2 consumers are new so have no previous tasks.
Then after the assignment the 2 new consumers would still have no tasks. But maybe we'd want to re-distribute the standby tasks across the 2 new consumers? I'm not sure. I'd welcome your thoughts.
Thanks

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