Consider moving routing logic into Action modules

[whitfin]

Currently we define routing styles inside Cachex.Router, but it would be easier to maintain if it's a module attribute of the actual Action (to keep things in one place). We should test and see if something like this is viable:

defmodule Cachex.Actions.Get do
  @routing :key
end

This would cause an extra lookup in the routing layer, but it's so minimal that I don't expect it to move things much. There are other routing improvements coming in 5.x that routing should still generally be a net improvement overall.