Sender invoke Fail Notification

[agarakan]

Description of the issue

Sender invokes batch.fail() callbacks when request fails and concurrency is enabled

Description of changes

1. Adds concurrency awareness to Sender
2. Adds Fail callbacks to Batch
3. Adds unit tests to ensure fail() is called when concurrency is enabled
4. Ensures existing functionality is the same for synchronous mode

License

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Tests

Added Unit tests pass

make lint
make fmt
make fmt-sh
make test

Requirements

Before commiting your code, please do the following steps.

1. Run make fmt and make fmt-sh
2. Run make lint

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Integration Tests

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