Promise interface

Been working with a lot of async/await lately and a callback API is really hindering there. I think we can return promises if the callback argument is not a function. Or should we go the route of completely removing the callback interface and do a major version bump?

Start by reviewing the existing callback API and the issue comments to understand how asynchronous DNS requests are exposed. Resolve whether promises should be added when the callback is not a function or whether the callback interface should be removed in a major version; done means the chosen approach is clearly specified and consistently supported.

Written by the indexing model from the issue text.