Extend Middleware to include the response from the database

[mvgijssel]

#119 needs to have access to the response of the SQL query, so it knows which rows are affected for a subscription.

Currently an Arel ast is passed in to a middleware class as an argument and the return value is the argument for the next middleware. This needs a way call the next middleware from within the middleware to be able to get access to the response.

Rack and Express both have patterns to do this. Express passed the next middleware as an argument while Rack sets the next middleware in an initialiser. Passing the next middleware as an argument has the benefit that the classes can be stateless:

class Middleware
  def self.call(arel, next_middleware, context)
    updated_arel = arel.query(class: Arel::Table).each { ... }
    next_middleware.call(updated_arel)
  end
end

Here the next.call will call the next item in the middleware. The return value of this call will contain the actual response of the database.