Limited probing of available DNS servers

[nyx191]

Suppose I'm using config like:

# as strict filters as possible, but there are still many hosts like that
dnscrypt_servers = true
doh_servers = false
odoh_servers = false

require_dnssec = true
require_nolog = true
require_nofilter = true

Its advantage is that we don't need to maintain explicit list of servers (which will be get outdated eventually). Disadvantage is that (short-living) client creates connections to many hosts on every startup which I think may create network noise comparable to total amount of connections system made until restart.

To mitigate that, we may want to somehow limit probed set of servers from public-resolvers.md further. Some viable strategies:

1. Take N random hosts, probe only them
2. Filter hosts by geoip

First one should be implemented easily without any new deps, but may lead to worse delays. Second option may potentially add a new dependency to a project.

Maybe it is already doable? Cannot find any related config entry