Major async refactor of 2020

This PR fuses alot of changes into a single asyncify of the entire code-base.

Unfortunately, I would have liked to split these set of changes into separate MRs, but it got out of hand and now I dont want to nest up in which feature changes where introduced and removed where, in the asyncify process.

Key takeaways for the changes are:

• Async the entire codebase.
• Test body is not a n async moved closed, making it possible to await within it.

test.run(|ops| async move {
  run_test.await;
});

• Added inject_container_name to Composition, giving you the possibility to communicate with other containers through configured environment variables. One cannot hardcode this information, since the final container name is generated by dockertest (to avoid parallel collisions).
• Removed host port mapping. Networking should only rely on the ip/container_name solution.

See the CHANGELOG.md for details affecting the public interface.

Closes #1 (closed), #4 (closed), #5 (closed), #6 (closed)