Bugfix - Cancel Race Conditions

[cnoon]

This PR is a follow-up to #368 and #371 that is more comprehensive. After digging in further with @jshier, I was able to create a set of stress tests to test all the odd edge cases including:

• Large image downloader queues
• Resuming queued requests by manually calling .resume() on the request through the request receipt
• Cancellation of queued requests using request receipts
• Cancellation of queued requests by cancelling directly on Request using the RequestReceipt: receipt.request.cancel()

These tests were able to reproduce somewhat reliably race condition logic errors in the previous logic.

Goals ⚽

1. Remove all logic where the ImageDownloader was relying on a task to be present when it can no longer be relied upon safely.
2. Remove logic where the ImageDownloader would early out in completion if the response did not contain a request used to cache the image when successful.

Implementation Details 🚧

These changes are simply removing the reliance on anything with request creation being synchronous. The stress tests were able to tease out the troublesome spots.

Testing Details 🔍

The big changes here are the stress tests. I've placed them directly in the test target for now since there are only 4 tests which hit the service 10 times each. Therefore, it's lightweight enough to not have to create a separate target.

These tests will break roughly 1 out of 4 tries when executing without the changes in the ImageDownloader. However, with these changes, I have yet to see any tests fail on AF beta 6 or 7.

I'm not completely convinced that there isn't an additional bug in Alamofire related to cancellation as well. I've spent a lot of time trying to track it down, but have not yet been able to 100% pinpoint the Alamofire logic as a contributor to the race. I'll continue to investigate further over the next couple of days to see if I can reliably reproduce the issue.

[kinarobin]

@cnoon Could you have a look for AFNetworking #4407 ? This is same issue.
Thank you very much!