Force retry if server responds with SERVFAIL

[jmthackett]

Hello!

First of all: this project is great. I came across it from the archlinux wiki and have started running it locally as my ISP appears to block DoT - let 'em try to block this!

I've been using the OpenNIC servers, which can be a touch unreliable. It can take a couple of queries for them to manage to get responses back. I realise this is an issue for them, really, but I'm wondering if doh-client might include configuration for retrying a specific number of queries with a time between them. Something like this:

[[upstream.upstream_ietf]]
    url = "..."
    weight = 50
    retry_attempts = 5
    retry_interval = 1

If it is a non-starter, don't worry - it is on me to choose more reliable servers, or maybe try to contribute to the OpenNIC project to solve my own problem. But if you can help it'd be great.

Cheers John