Request: Expose is_disconnected() on Sender and Receiver

[BruceBrown]

Currently, there's no way for a receiver to determine if it is disconnected, other than to try and receive something from the channel, which you don't always want to do. In my case, I want to check is_empty() and schedule a task if the receiver isn't empty. However, is_empty(), returns true if the queue is empty and it has been disconnected, leaving me with a dangling channel that doesn't get cleaned up. I don't think having is_empty() return false for a disconnected channel works, as that's going to make len() a bit odd. While I could use select, I've found that doesn't work so well, with 1000s of channels.

[taiki-e]

related: #236, #497