Add integration tests for durable requests

Original issue: https://github.com/channable/icepeak/issues/25
Associated PR: https://github.com/channable/icepeak/pull/27

This feature has currently no integration tests. We should write some.

Idea from @fatho to test this:

> You could spam it with updates to some key /foo/a and then a last update to /foo/b, then ask for the value of /foo/b. If processing all the updates takes long enough, it should sometimes return the old, sometimes the new value for /foo/b without the flag. But with that flag, it should always return the updated value.

Alternatively, we could perhaps also add some artificial delays in the processing of requests with eventual consistency. This would make the tests more deterministic, but it would clutter the code a bit.