Multi-instance test case

[Antman261]

Outcome:
Prove es-reduxed works with multiple instances

Implementation

• Create a docker-compose config for testing multiple instances, i.e. 1 postgres instance and 8 instances of a server running es-reduxed
• Create a postman script to smash these 8 instances with GET and POST requests, ensuring that state updates across all 8 instances following a POST

This test implementation will be useful when adding additional provides, such as MongoDB. We should be able to swap out different event stores and maintain the same behaviour.