Previously if dns.lua failed to resolve a name you'd see the following in your logs:
```
2019/10/12 23:39:34 [error] 41#41: *6474 [lua] dns.lua:121: dns_lookup(): failed to query the DNS server:
server returned error code: 3: name error
server returned error code: 3: name error, context: ngx.timer
```
Unfortunately this doesn't tell you what name is failing (so you have to start guessing). To alleviate the pain this simply adds the host name we are attempting to resolve to the log line so users don't have to guess.
It seems that when support was added for parsing resolv_conf directly a regression was introduced which effectively breaks anyone with ipv6 resolvers.
Regression of #3895
1. Session cookie is updated on previous attempt failure when `session-cookie-change-on-failure = true` (default value is `false`).
2. Added tests to check both cases.
3. Updated docs.
Co-Authored-By: Vladimir Grishin <yadolov@users.noreply.github.com>
