Cancel redirects for the same requestIds and urls if originating from the same tabId

[stoically]

As mentioned in #1114 there are still cases where two tabs are opened when redirects are involved:

In this case www.youtube.com is assigned. The initial request goes to http://youtube.com which results in redirects to https://youtube.com and two times https://www.youtube.com - the last request has a different requestId. All have the same tabId.

To fix this all requestIds and urls that reach the point of being "reopened" are marked as canceled in relation to the tabId - or are canceled if already marked as canceled. Instead of cleaning up canceled requests with a timeout, we can use webRequest.onCompleted and webRequest.onErrorOccurred - though, since they are not 100% reliable, a 2seconds backup cleanup timer is included (uncanceling is also covered with tests).

The downside of this fix is that, if the user left clicks http://youtube.com on a website and switches back to the website, while the confirm page or target container is loading and the completed callback, error callback and backup cleanup timer haven't fired, then all clicks in that tab on the urls marked as canceled will be canceled. I'd personally say that is highly unlikely - the requests complete normally really fast, unless the user has a really slow internet-connection, and even if not after 2seconds the requests get uncanceled - and still preferable in comparison to two tabs opening at random.

Fixes #940 #1003 #1080 #728 #912

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Some informations when and how the redirects happen:

First time with clean Network cache in Firefox. 3 requests, all have the same requestId

[requestId: 1] http://youtube.com -[network]-> 301 - onBeforeRedirect fires
[requestId: 1] https://youtube.com -[network]-> 301 - onBeforeRedirect fires
[requestId: 1] https://www.youtube.com

Second time with filled Network cache in Firefox, 4 requests, last has a different requestId

[requestId: 1] http://youtube.com -[internal]-> 301 - onBeforeRedirect doesn't fire
[requestId: 1] https://youtube.com -[network]-> 301 - onBeforeRedirect fires
[requestId: 1] https://www.youtube.com -[internal]-> 301? - onBeforeRedirect doesn't fire
[requestId: 2] https://www.youtube.com

Filed a Firefox bug since I believe this is not intended behavior. If the requestId wouldn't change or we had a way of correlating the two requests, this fix could be made side-effect-free.

[groovecoder]

I had trouble reproducing the original bug in master, but I know I've seen it before. This code looks good to me, and I tested this branch with no problems.