Pebble accepts a `--dnsserver` flag, that allows to select a custom
DNS resolver to use when validating the ACME challenges. This option
is critical to make the certbot integration tests work in particular.

Current implementation is to set the `net.DefaultResolver` on a new
`net.Resolver` instance with a custom `Dial` property to consume the
value from `--dnsserver`. This allows to transparently resolve any
host using this custom DNS resolver from a pure-Go implementation of
the DNS resolution API.

Sadly this approach does not work on Windows, and the `--dnsserver`
has no effect on how resolution is done, and it is always the OS
mechanism that is used.

One can reveal this behavior by running the following piece of code on
Linux and on Windows:

```go
// test.go
package main

import (
	"context"
	"fmt"
	"net"
)

func main() {
	resolver := &net.Resolver{
		PreferGo: true,
		Dial: func(ctx context.Context, _, _ string) (net.Conn, error) {
			d := net.Dialer{}
			return d.DialContext(ctx, "udp", "4.3.2.1:404")
		},
	}
	net.DefaultResolver = resolver
	ctx, cancelfunc := context.WithTimeout(context.Background(), 30)
	defer cancelfunc()
	_, err := resolver.LookupTXT(ctx, "stupid.entry.net")

	fmt.Printf("Error is: %s\n", err)
}
```

This piece of code tries to resolve a non-existent domain on a
non-existent DNS server as IP `4.3.2.1:404`.

On Linux, you will get the following error:

```bash
Error is: lookup stupid.entry.net on 127.0.0.53:53: dial udp 4.3.2.1:404: i/o timeout
```

That indicates that the system tried to reach the non-existent DNS server,
and get back a timeout exception on it.

However on Windows you will get:
```bash
Error is: lookup stupid.entry.net: dnsquery: DNS name does not exist.
```

This indicates that the system ignored the dummy DNS server address,
contacted the OS DNS resolver, that responded that the DNS name does
not exist.

One can see also the reason for this behavior on Windows on the `net`
godoc, https://godoc.org/net, in particular this line in the module
introduction:

```
On Windows, the resolver always uses C library functions, such as GetAddrInfo and DnsQuery.
```

In fact, the pure Go DNS resolver is not applicable on Windows, the OS
DNS resolver will be used, ignoring any customization.

Several relevant discussions, in particular a proposal (not developed yet)
to make the pure Go DNS resolver available on Windows:

* golang/go#29621
* golang/go#33097
* golang/go#33086

To fix this, this PR makes Pebble switch to a different logic:
* if `-dnsserver` is not set, use the default API to resolve the names
* if `-dnsserver` is set, use a dedicated DNS client, to avoid to use
the OS one both on Linux and Windows

The DNS client is https://github.com/miekg/dns, a highly used and
supported DNS library.

With these modifications, integrations tests on Certbot are running
correctly both on Linux and Windows.