feature/refactor-inflight-messages

[dginzbourg]

Resolves #NUMBER

Description:
Implement asynchronous message processing and polling up to maxInflightMessages.
graceful shutdown termination was added to await inflight messages be consumed.

Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• [* ] Breaking change (fix or feature that would cause existing functionality to change)

Why is this change required?:
The original library doesn't support asynchronous messages processing and polling to buffer the messages

Code changes:

• Consumer calls polling and creates asycn handlers to process messages
• Polling is done until maxInflightMessages reached, then it perorms timeout waiting until inflight messages number is reduced

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Checklist:

• [+] My code follows the code style of this project.
• [+] My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.