Don't use the system default resolver implicitly

[ameshkov]

There're a couple of places in the code where we use system default resolver implicitly:

• Downloading filters updates
• Communicating with the browsing security and parental control web services
• Checking application update

Ideally, we should avoid doing it. But I don't like the idea of propagating the custom resolver to every place where we make a http request. Can we somehow change the default resolver globally?

[szolin]

May be relevant: golang/go#12503

[ameshkov]

Here's what I see on first run:

2019/04/23 19:01:35 [info] Couldn't request filter from URL https://adguardteam.github.io/AdGuardSDNSFilter/Filters/filter.txt, skipping: Get https://adguardteam.github.io/AdGuardSDNSFilter/Filters/filter.txt: lookup adguardteam.github.io on 192.168.10.125:53: read udp 127.0.0.1:57264->127.0.0.1:53: read: connection refused
2019/04/23 19:01:35 [info] Failed to update filter https://adguardteam.github.io/AdGuardSDNSFilter/Filters/filter.txt: Get https://adguardteam.github.io/AdGuardSDNSFilter/Filters/filter.txt: lookup adguardteam.github.io on 192.168.10.125:53: read udp 127.0.0.1:57264->127.0.0.1:53: read: connection refused

We shouldn't load filters unless AGH DNS server is running.

[ameshkov]

And here's the second issue: handle DNS host=0.0.0.0 -- use 127.0.0.1 in this case.

[ameshkov]

Fixed

[ameshkov]

I had to roll it back due to the Docker-related issue:
#730

See some details here:
https://stackoverflow.com/questions/24319662/from-inside-of-a-docker-container-how-do-i-connect-to-the-localhost-of-the-mach/52858101

Also, I just realized that we're passing resolverAddr via global variable and that can be done better.

[ameshkov]

False alarm :)