Provide a centralised point for accessing app configuration

[stuartc]

We currently have some 'sane' defaults like :adaptor_service_name etc, for looking up the Adaptor service process. However changing it for any reason makes things pretty difficult to track.

Oban has some lean and useful examples on how to deal with this:

• A thin abstraction over Registry providing some conformed helpers to generate :via tuples.
  https://github.com/sorentwo/oban/blob/master/lib/oban/registry.ex
• Which is used in (for example) their listener module

  def listen(oban_name, [_ | _] = channels) do
    oban_name
    |> Registry.whereis(__MODULE__)
    |> listen(channels)
  end

• And then started via the application supervisor

I think we can benefit significantly by adopting this kind of pattern, our lack of centralized app config is getting unwieldy.