Add capability to load test with one click

[marein]

This includes provisioning a configurable number of servers at a cloud provider, such as DigitalOcean, and creating scripts that, for example, create random chat messages or play games against each other. The application should be deployed using Docker Swarm. The scripts should have a default configuration that is able to produce 18k write requests per second. I've already created everything locally and used them to identify issues such as #98, #104, #105, #106, #114 -> #185 for scaling queues to achieve higher message throughput, #115, #119, #173 and #225. It's useful to push them into the repository after a cleanup. Maybe a draft pull request is good enough for now.

The old script for load tests should be removed as it doesn't scale and produces lots of TCP connections in TIME_WAIT state due to its implementation using file_get_contents.

The documentation should be updated to point to this example, e.g. the section "Scale-Out the application".