Migrate away from elixir-dns

Currently, IBDNS relies on the elixir-dns library.

However, it has several issues:

• it relies on inet_dns, an Erlang module which is actually an implementation detail for Erlang/OTP’s built-in resolver. It isn’t meant to be used as a library for use by the general public;
• consequently, it has bugs that depend on the version of OTP that’s installed on the developer’s machine;
• furthermore, it has shortcomings of its own.

A possible option to consider is dns_erlang.