Introduced new flag: 'autoclose_cookie_session'.

This flag lets the cookie session to be disposed by *CookieManager* when the cookie session is no longer used.

I had a memory issue when working with a spider which yields lots of requests with a unique cookie session in a loop where those cookie sessions had only one-shot usage. Eventually and inevitably, scrapy process hits the memory boundary much earlier.

This PR introduces a new flag, **autoclose_cookie_session** which could be set in 'meta' of the Request.

Behind the scenes, every request which set this flag is tracked and when all of the tracked requests are released, the relevant cookie session is deleted.