future improvements and issues found in my reworks

[Luap99]

A list of problems or possible enhancements I found while reworking parts of aardvark-dns

• resolv.conf is read only once and not refreshed (also there is no cache of known bad upstream servers so we always try the broken resolver making problems like DNS server fallback not working #482 much worse)
• there is no caching of upstream dns requests
• better error handling wrapping and message, the current message stutter a lot. We should convert everything to the new AardvarkError type and make sure the error message make sense
• add handling of MX like docker does
• check performance of the server and possible improvements currently we process only one tcp or udp message from the same network at the same time, i.e. if we parse a udp message we will not accept another tcp/udp message in parallel until we progressed the current message (do not include external forwarding). Maybe it is good that way, because it doesn't allow to flood the server with requests easily.
• In general use better types that are passed around. There is a lot of random string parsing at many places that could have been just proper types.

[Luap99]

cc @mheon

Not sure if you care much about any of them.

[mheon]

I think the first bullet point is rather important, that is something we'll want to respect.

I think the rest are lower priority, save maybe performance? But we don't have data there to say that things aren't as fast as they need to be.