Fix metadata order in response when producing records

[robobario]

fix: preserve record send order in response metadata

This uses the send CompletionStages to keep the original ordering of records.

kafka-clients KafkaProducer#send only guarantees that callbacks are executed in order per partition, the ordering across partitions is not guaranteed.

Callbacks for records being sent to the same partition are guaranteed to execute in order. That is, in the following example callback1 is guaranteed to execute before callback2:

producer.send(new ProducerRecord<byte[],byte[]>(topic, partition, key1, value1), callback1);
producer. send(new ProducerRecord<byte[],byte[]>(topic, partition, key2, value2), callback2);

Closes #944

[robobario]

the ppc64le build failed with:

SeekIT.seekToOffsetMultipleTopicsWithNotSuscribedTopic(VertxTestContext) java.lang.AssertionError: 
Expected: is "No current assignment for partition seekToOffsetNotSubscribedTopic-0"
     but: was "No current assignment for partition seekToOffseSubscribedTopic-0"

doesn't sound related to produce changes

[ppatierno]

LGTM! Good catch @robobario! Thanks!

[scholzj]

LGTM if it works.

[ppatierno]

I am going to cherry-pick this one for the release-0.31.x branch for having it in the 0.31.0 release.