[Issue 367][producer] Send delay message individually even batching is enabled

[faycheng]

Fixes #367

Motivation

Send delay message individually even batching is enabled.

Modifications

1. flush batching messages immediately when a new delay message is received
2. reset deliverAtTime metadata of BatchBuilder

Verifying this change

Ref #367

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): (no)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: (no)

Documentation

• Does this pull request introduce a new feature? (yes)
• If yes, how is the feature documented? (docs)
• 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

[wolfstudy]

@faycheng The change LGTM+1, can you add a test case for this change?

[faycheng]

@faycheng The change LGTM+1, can you add a test case for this change?

Sure.I will add a unit test.

[wolfstudy]

LGTM +1