[pulsar-client] Remove UUID generation on sending message

[rdhabalia]

Motivation

Pulsar client producer requires uuid while sending chunked messages. right now, Pulsar-client lib generates UUID for every send message which is expensive and impacts publish performance. Therefore, UUID generation shouldn't be expensive and producer should not generate uuid for non-chunked messages.

Modification

• Avoid uuid generation for non-chunk messages
• Generate uuid for message using global-producer name and message-sequenceId.

Note
This fix should be cherry-picked to 2.6.1 release as well.

[merlimat]

This will always be the same as producerName. If the client overrides it, the server will use that name, but if a producer with same name is already connected, it will instead error it out

[rdhabalia]

I have changed it but I don't think user_producer_id + sequence_id can create UUID because user_producer_id can be reused by other producer later on and sequence_id can start with 0 again and it will create a duplicate combination which will not be unique across messages on the topic and that can be issue for message chunking.
eg:
Producer P1 gives name userProducer1 and publish message with sequenceId=0. Now that process died and another process created producer with similar name userProducer1 and started message with sequenceId=0 in which case UUID will not be unique.
Therefore, using serverProducerName always gives a guarantee for unique producer-id and UUID. Adding new string serverProducerId will not create any overhead as well. So, can you please let me know your thought on it.

[rdhabalia]

@merlimat did you get chance to read my above comment before merging the PR?