resolved: add new helper for carefully detach a stream from any server

resolved: add new helper for carefully detach a stream from any server

This adds a helper call for detaching a DnsServer from a DnsStream if
the latter is the "default" stream of the server.

Also, let's unref the stream in dns_stream_stop() rather than
dns_stream_free(): as soon as our stream is disconnected by stopping
there's really no need to keep it as default stream for the server
around.

Since dns_stream_free() calls dns_stream_stop() we can remove it from
the former.