Make response handler management more flexible

Make response handler management more flexible

Extend AsyncDnsServer.install_response_handler() so that the provided
response handler can be inserted at the beginning of the handler list.
This enables installing a response handler that takes priority over all
previously installed handlers.

Add a new method, AsyncDnsServer.uninstall_response_handler(), which
enables removing a previously installed response handler.

Together, these two methods provide full control over the response
handler list at runtime.