[FLINK-16383] Set close = true in monitor/lock scope in KafkaProducer

[aljoscha]

Before, the flag was changed outside the lock, which would allow the
case that the flag is set to true while someone else is holding the
"close lock".

[becketqin]

Thanks for the patch, Aljoscha. LGTM.

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[aljoscha]

@becketqin I added some debug logging.

[StephanEwen]

Good with one minor comment.

[StephanEwen]

This uses eager string concatenation in a lazy parameter, which is typically a blocker.
Now, this is not a performance critical path, but I would still suggest to avoid this, because it sets a bad example for other contributors to immitate (in then more critical parts).

[aljoscha]

Yes, that's why I left it there but I agree that we should set good examples? So I should put an if (LOG.isDebugEnabled()) {} around it?

[aljoscha]

Thanks for the swift reviews!