Add AwaitWithStopOnCancellation

[bartelink]

The current API does not provide a clear way to have a service cleanly shut down in response to the faulting of one of a set of consumers.

This PR extends the API to provide such a mechanism

[bartelink]

Will likely merge these changes in, along with #73 as part of the 1.6.0 release (when the associated underlying Confluent.Kafka release happens)

For now, this extension can be approximated via this helper:

[<AutoOpen>]
module ConsumerExt =

    type Consumer with
        member consumer.AwaitWithStopOnCancellation() = async {
            let! ct = Async.CancellationToken
            use _ = ct.Register(fun () -> consumer.Stop())
            return! consumer.AwaitShutdown()
        }

See https://gist.github.com/bartelink/14b7a1474e5376b59e62ddf8caacdb44 for a testbed