added closed connection callback

[lashket]

Added callback to be notified when Websocket is closed

[cryptpa]

you are the man!!! What about closing the request stream connection.

[marinkobabic]

Why is this pull request still open?

[szymek-pindelski]

hello,
does this pull request have something to do with active stream subscriptions surviving RSocket.close() ?
My stream subscription does not seem to notice that rsocket has been closed, and I need to explicitly cancel it (in case you're wondering - I'm using websocket as transport layer)
I read somewhere that RSocket.close() is supposed to cancel any active subscriptions but it's clearly not the case with RSocket Dart implementation