[fix] Fix ordering key not being set and parsed when batching is disabled

[RobertIndie]

Motivation

When batching is disabled, the ordering key couldn't be configured and parsed correctly. This is a regression bug.
For the reproducible code, please see the test TestNoBatchSendMessagesWithMetadata in this PR.

Modifications

• Set the ordering key when publishing a single message
• Parsed the ordering key when consuming a single message

Verifying this change

This change added tests: TestBatchSendMessagesWithMetadata and TestNoBatchSendMessagesWithMetadata

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

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API: (yes / no)
• The schema: (yes / no / don't know)
• The default values of configurations: (yes / no)
• The wire protocol: (yes / no)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / GoDocs / not documented)
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation