Expose BatchingMaxSize from ProducerOptions

[dferstay]

Previously, the producer maximum batch size was hard-coded to 128 KB.

Now, the produdcer maximum batch size is exposed via ProducerOptions
and defaults to 128 KB

Motivation

Different applications producing to Pulsar require different batching policies.
Previously, users had control over the maximum batching delay and the maximum number of messages in a batch, but could not control the maximum number of bytes in a batch. This change enables the latter.

Modifications

Expose a new ProducerOption.BatchingMaxSize (default: 128KB) that allows the user to configure the maximum batch size. This option has been plumbed through partition_producer module to the internal/batch_builder module where it is used.

Verifying this change

• Make sure that the change passes the CI checks.

This change is already covered by existing tests, such as producer_test.TestFlushInPartitionedProducer and producer_test.TestFlushInProducer.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• The public API: The pulsar.ProducerOptions structure gains a new field: BatchingMaxSize

Documentation

• Does this pull request introduce a new feature? Yes
• If yes, how is the feature documented? GoDocs