Extend Phoenix.Ecto.SQL.Sandbox module for testing

[btkostner]

The Phoenix.Ecto.SQL.Sandbox module is amazing for testing applications when you are utilizing the database. However, this kinda falls apart when you start integrating messaging systems like rabbitmq. It also requires sending a request to start the pool which can be hard to do in distributed system.

Things we can do

1. Document and make the module easier to use.
2. Possibly a -test tag docker image to replicate use in distributed system CI systems
3. Use similar logic for message bus. Sending and receiving rabbitmq messages should be done in the same db pid
4. Investigate ability to not require sending a POST request to initiate the request. This will help in distributed systems