[DISCUSS] Should Apache APISIX use local DNS resolver by default?

[YuanSheng Wang]

Hi:

I think it is time to use the local DNS resolver by default. This is
more suitable for most users,
especially for k8s users.

```yaml
apisix:
  # dns_resolver:                   # If not set, read from `/etc/resolv.conf`
  #  - 1.1.1.1
  #  - 8.8.8.8
```

Here is the default value for DNS resolver now.

```yaml
apisix:
  dns_resolver:                   # If not set, read from `/etc/resolv.conf`
    - 114.114.114.114
    - 223.5.5.5
    - 1.1.1.1
    - 8.8.8.8
```

-- 
MembPhis
My github: https://github.com/membphis
Apache APISIX: https://github.com/apache/incubator-apisix