Refactor ControllableAsyncDnsServer setup

Refactor ControllableAsyncDnsServer setup

When this class was introduced, the constructor of its base class had no
parameters. This was changed in the meantime and these parameters were
not accessible by users of the subclass.

Don't override the constructor.
Move command setup to methods.
Move subclass-specific storage to cached properties.
Take instances of Command instead of the classes themselves for
symmetry with install_response_handler.

(cherry picked from commit a0970f3d0410afd35799306c24f8ed43955ad027)