Execute KafkaProducer#send calls directly and not on the blocking thread pool

[svroonland]

See #531

[iravid]

I think this is generally fine but just want to say that in catastrophic cases where the broker is down (for example) and the producer buffer fills up, calls to send most definitely will block. I think this is documented in the Java API.

[svroonland]

Right, found it:

When the buffer space is exhausted additional send calls will block. The threshold for time to block is determined by <code>max.block.ms</code> after which it throws a TimeoutException.

Thanks!

[guizmaii]

Can't we bypass this mechanism from the Java client by implementing our own buffer and using a non-blocking "blocking mechanism"? Might be a crazy idea 🤔😅

[svroonland]

I suppose we could yes, but it's quite some effort for an exceptional case. Feel free to do so of course! :)