refactor: refactor DNS resolution

This commit also adds some of tests for the Domain model. It was during the writing of these tests that the DNS resolution refactoring requirement became apparent.

spec/lib/postal/received_header_spec.rb

@@ -4,7 +4,7 @@ require "rails_helper"
 
 describe Postal::ReceivedHeader do
   before do
-    allow(Resolv::DNS).to receive(:open).and_return("hostname.com")
+    allow(DNSResolver.local).to receive(:ip_to_hostname).and_return("hostname.com")
   end
 
   describe ".generate" do