* use HttpContext.RequestAborted to support cancel a request
* register context.HttpContext.RequestAborted.ThrowIfCancellationRequested() in ExceptionHandlerMiddleware
* add LangVersion
* add default number to OcelotErrorCode
fix unit test
* revert back to one line
* added delegate to select last handler
* #529 implemented a way we can inject the last delegating handler
* wip - moving code
* #529 removed loads of qos code and moved it into Ocelot.Provider.Polly
* #529 can now inject http client expcetions to ocelot errors mappers and updated docs