Crawlera retry delay

In this PR I've changed 2 things:

- If middleware detects Crawlera deploy (ConnectionRefusedError) - it adjusts slot download delay until spider gets response from Crawlera.
- Previously original slot delay was never restored - it was set to `0` after a successful response. Now original delay is saved and restored each time spider gets response from Crawlera.
Since `ConnectionRefuseError` is always because a Crawlera reset, returning the request will be ok, but you can add limit to that retry for mayor security. Having a function provided by `Scrapy` would be nice for these cases, and when you do some kind of handling from the spider.