Handle Redirects one by one with scrapy
I found my own solution to this promlem. Instead of building a list of requests and return them at once, I build a chain of them and passed the next one inside the requests meta_data. 
Inside the callback I pass either the next request, storing the parsed item in a spider member, or the parsed list of items if there is no next request to execute.
I've written a Spider which has one start_url. The parse method of my spider scraps some data and returns a list of FormRequests. 
The problem comes with the response of that post request. It redirects me to another site with some irrelevant GET Parameters. The only parameter which seems to matter is a SESSION_ID posted along in the header. Unfortunately Scrapys behavior is to execute my requests, one after another and queues the redirect response at the end of the queue. If all returned FormRequests are executed, scrapy starts to execute all redirects, which all return the same site. 
How can I circumvent this behavior, so that a FormRequest is executed, and the redirect returned in the requests response is executed befor any new FormRequest? Maybe there is another way, like forcing the site somehow to get a new SESSION_ID cookie for each FormRequest. I'm open to any idea that could probably solve the problem.