Write more integration/E2E tests

[rainerschoe]

This shall be a list of tests we think makes sense based on our initial review of the code and some refactoring:

• Figure out how to run E2E tests against a production-like server
• If the connection with the web socket is lost, are we recovering once is available again?
• Once reconnected, are receiving all the pending updates?
• If the connection drops, do we keep running and effectively using the Cache?
• handle temporary connectivity issues / errors
• I return the correct error to the user if credentials are wrong
• I return the correct error to the user if the feature flag doesn't exist

[rainerschoe]

From Juan's suggestions on PRs #2 #3 and #4:

Unit tests:

• Are we spawning one thread, and only one, per every AppConfig client?
• Are we correctly terminating the thread when AppConfig goes out of scope?

Acceptance Tests:

• I'm able to get a feature sucessfully
• Does a change in the a feature flag/Property/elsewhere(?) get populated to the client?

Integration tests:

• If the connection with the web socket is lost, are we recovering once is available again?
• Once reconnected, are receiving all the pending updates?
• If the connection drops, do we keep running and effectively using the Cache?
• handle temporary connectivity issues / errors
• I return the correct error to the user if credentials are wrong
• I return the correct error to the user if the feature flag doesn't exist