Instantiate middleware

[ChrisBr]

So far we only created one instance of the middleware and then operated on this instance. This made the code quite ugly because we always needed to pass around the method parameters from method to method. This PR creates now a new instance for each request.

The main part what I changed is basically from

class Subscriber
  def call(name, start, finish, payload)
  end
end

to

class Subscriber
  def self.call(name, start, finish, payload)
    new(name, start, finish, payload)
  end
end

which resulted that we basically dropped all method parameters from all functions. In my opinion this makes the code a lot nicer to work with because we can easily extract helper methods and memoize values. This refactoring is a preparation for #117 which made it quite hard to squeeze more code without the ability to have helper methods.

Downside of this approach is of course that we create a few more instances per request but I think this is worth it.

[hennevogel]

I don't think the subscribe_to method brings any clarity anymore.

[ChrisBr]

Yeah good point 👍 Lets get rid of it!