SshChannel leak via long living cancellation arg of OpenChannelAsync()

[IlyaBiryukov]

SshSession.OpenChannelAsync() registers a callback on its cancellation argument like this:

if (cancellation.CanBeCanceled)
{
	cancellation.Register(() => completionSource.TrySetCanceled());
	cancellation.ThrowIfCancellationRequested();
}

The callback registration is never disposed of.

If OpenChannelAsync() is successful, completionSource will reference the opened SshChannel and the SshSession. If the cancellation argument is long living, it'll root them after OpenChannelAsync() is done, potentially causing a memory leak.

There are several other places in C# SSH library with a similar pattern.