We now enforce the fact that a query always include a question.
It holds true for all practical use cases of dnscrypt-proxy.
This avoids quite a lot of redundant code in plugins, and is faster.
DNS return codes are not enough; we need to change this to something
more expressive.
In particular, we can't use them to distinguish between a server block,
a blacklist block, and a plugin block such as the IPv6 blocker.