Add Closed event to ShellStream.

[scott-xu]

Lib consumer could hook to this event to detect if channel is closed by server in time.
Closes#44

[jscarle]

Can you please link #44 in your description to close it with this PR?

[scott-xu]

In our case, for some reason, the server may send channel close message to the client. As the client, we want to know this event in time and do some follow up work.
I checked the existing unit tests but no related cases found.

[scott-xu]

Can you please link #44 in your description to close it with this PR?

I should search first. Anyway I'm not alone. Thanks!

[Rob-Hague]

What's the use case here?

Now that Read blocks until the channel is closed, can you just wait until Read returns 0?

[scott-xu]

What's the use case here?

Now that Read blocks until the channel is closed, can you just wait until Read returns 0?

We don't call Read. We use Write method and DataRecieved event.
You can check my previous comment for use case.

[WojciechNagorski]

I think we can merge this PR. Fits the current SSH.NET architecture.