Do not acquire KafkaClient lock for read-only operations

[dpkp]

In an effort to reduce the surface area of lock coordination, and thereby hopefully reduce lock contention, I think we can remove locking from the read-only KafkaClient methods: connected, is_disconnected, in_flight_request_count, and least_loaded_node . Given that the read data could change after the lock is released but before the caller uses it, the value of acquiring a lock here does not seem high to me.

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This change is 

[jeffwidman]

Makes complete sense, and the implementation looks good.

I would probably copy your PR description text (which is quite helpful) into your commit message as well, so that it's easier for someone spelunking git blame in the future.