Remove custom cancellation token handling code, rely on Stream.ReadAsync to handle it instead

[danzel]

Fixes #131

I've tried tracing this code back in git history to figure out why this override exists. Previously it had a comment:

We ensure that this service can be closed ASAP with the help of a Task.Delay.

Does Stream.ReadAsync take meaningfully longer to cancel than using a TaskCompletionSource?.

If there is a reason it is this way, we can probably keep the behaviour and observe the result so that the we don't end up with an UnobservedTaskException later, it'll be a bit clunky though.

[fubar-coder]

It might've been a problem in an older version of .NET (Core), that the stream would only be cancelled when new data arrives or some timeout occurs.

[danzel]

Is this something we need to support or test? If you can let me know what needs manual testing I can hopefully do it.

[fubar-coder]

I thought that I read in an article (MS blog post), that they fixed it in .NET Core 3.1, but I might be misremembering it. You can read more about it here:

https://devblogs.microsoft.com/dotnet/performance_improvements_in_net_7/#file-i-o