This patch refactors the DNS default resolver code to make it
easier to be included in a snapshot:

- The code specific for the callback-based DNS resolver are not
  in a separate module to make the dependency clearer (it's not
  actually needed if the user only ever loads `dns/promises`)
- The common part of the callback-based resolver and the promise-
  based resolver is now ResolverBase. The other two user-facing
  resolvers are now subclasses of ResolverBase. The default
  Resolver is constructed with just ResolverBase. This would
  be fine as the default resolver is never actually exposed
  to the user-land and it has been working using duck-typing anyway.
- Move the construction of Resolver subclasses into a common
  method `createResolverClass()` to reduce code duplication.
  The two subclasses now also share the same base constructor.
  This would make it possible for them to also share code
  for snapshot support later.
- `--dns-result-order` is now queried and refreshed during
  pre-execution. To avoid loading the cares_wrap binding unnecessarily
  the loading of the binding is also made lazy.