Return if a channel is on the process of being closed.
If a channel is being closed means no more message MSG can be sent over it. But, actually more message can still be received (and these message must be replied). On this context a channel being closed can be used to send rpy and err message.
This function can be used by vortex library consumer to know if a channel can be used to send more replies to received messages.
|channel ||the channel to operate on|
- if channel is being closed.