Remove callback severance on 5-to-3 adapter

[bretambrose]

1. Strip out everything related to the callback firewall on shutdown
2. Add a vtable to adapter operations and a fail function for publish, subscribe, unsubscribe
3. Invoke fail on all incomplete operations during final adapter shutdown
4. Add a test that verifies completion callback invocation after the adapter is shut down

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (0cc50d1) 82.34% compared to head (4f5f9a9) 82.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #329 +/- ## ========================================== + Coverage 82.34% 82.38% +0.04% ========================================== Files 20 20 Lines 8738 8721 -17 ========================================== - Hits 7195 7185 -10 + Misses 1543 1536 -7 ``` | [Files](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/329?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs) | Coverage Δ | | |---|---|---| | [source/v5/mqtt5\_to\_mqtt3\_adapter.c](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/329?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#diff-c291cmNlL3Y1L21xdHQ1X3RvX21xdHQzX2FkYXB0ZXIuYw==) | `76.33% <92.68%> (+0.18%)` | :arrow_up: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/329/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs)

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