DUP: move AMQP handling logic to Mississippi

[Annopaolo]

• [x] I have read CONTRIBUTING.md and CODE_OF_CONDUCT.md
• [x] I have added to CHANGELOG.md relevant changes and any other user facing change
• [x] I have added or ran the appropriate tests

What this PR does / why we need it:

Use the Mississippi framework to manage messages coming from VerneMQ. This allows to retain the same ordering and delivery guarantees, while moving to the external component all the intricacies of handling messages in a distributed setting. This will also increase the ease of contributing to the core message handling logic, as now code only works on what to do once a message is arrived, instead of also including how to receive a message.

Note that e2e tests will fail until https://github.com/astarte-platform/astarte_vmq_plugin/pull/94 is merged.

Which issue(s) this PR fixes:

(none)

Special notes for your reviewer:

Does this PR introduce a user-facing change?

• [ ] Yes
• [x] No

Additional documentation e.g. usage docs, diagrams, etc.:

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 69.35%. Comparing base (d9bc432) to head (7949de7). Report is 25 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #995 +/- ## ========================================== + Coverage 68.46% 69.35% +0.88% ========================================== Files 282 275 -7 Lines 7488 7149 -339 ========================================== - Hits 5127 4958 -169 + Misses 2361 2191 -170 ```

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