DNS Resolver: support both dnspython before and after 2.0.0

DNS Resolver: support both dnspython before and after 2.0.0

`dnspython` 2.0.0 has many changes and several deprecations like:

```
> dns.resolver.resolve() has been added, allowing control of whether
search lists are used. dns.resolver.query() is retained for backwards
compatibility, but deprecated. The default for search list behavior can
be set at in the resolver object with the use_search_by_default
parameter. The default is False.

> dns.resolver.resolve_address() has been added, allowing easy
address-to-name lookups.
```

The new class `DNSResolver`:
- provides the compatibility layer
- defaults the previous behavior (the search list configured in the
  system's resolver configuration is used for relative names)
- defaults lifetime to 15sec (determines the number of seconds
  to spend trying to get an answer to the question)

The compatibility shim was developed by Stanislav Levin for FreeIPA and
adopted for Samba by Alexander Bokovoy.

Signed-off-by: Stanislav Levin <slev@altlinux.org>
Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>