cancellation token support and http time out retry policy

[baywet]

Hi @kenakamu , @2toLeadMike and myself put together this pull request. it's rather big even if a big part of the changes are only exposing new methods and relaying the cancellation token. That PR brings two major improvements:

• ability to pass the cancellation token all the way down, so if the process is interrupted for some reason, all network requests that are ongoing will be as well
• implemented retry policy for http timeout (which throws a task canceled exception for some reason https://stackoverflow.com/questions/10547895/how-can-i-tell-when-httpclient-has-timed-out)

I understand it's a rather big PR, and we'd need this PR to be reviewed, merged and the nuget to be published fairly quickly if possible. We're approaching production deployment for one of our major projects. Thanks! And let me know if you have any question. :)

[kenakamu]

As this includes so many fixes which is nothing to do with cancel token, i prefer to have separate PR. However, as this is kind small repo anyway, i do review them all and accept as soon as possible.

[kenakamu]

@baywet Do you need to make method with cancellation token to public when we redirect call from original one? If we have plan to call it directly from outside, it makes sense.

[baywet]

Yeah the idea is to provide the ability to people to provide their cancellation token. Let's say I'm building a web API and the http call gets cancelled. It should be able to pass the cancellation all the way down to stop all the sub http calls the API was making to get the data.

[kenakamu]

merged