[Feature request] primary server and backup server

[Gontier-Julien]

It would be nice to have an option on the upstream DNS server selection, to have a primary server and others servers setup as backup in case the primary one fail ^^

It would be useful instead of doing a setup like:

Doh server ==> nginx load balance ==> local (or non-local) DNS server

So kinda something like this:

Doh server ==> if primary DNS server available then use it, if it down go to the backup one

[m13253]

Duplicate of #100.
Mainly delayed due to lack of development resource.
PRs are welcome.

Edit:
Sorry, I meant duplicate of #119.
An advanced selector engine in the future can measure the availability of servers.