Return channel opened state.
This is actually used by vortex_reader thread to detect if an incoming message received can be delivered. Vortex reader use this function to detect if a channel is being closed but waiting for some received msg to be replied after sending close message.
Before issuing a close message to close a channel (vortex_channel_close) the channel must fulfill some steps. So, until reply to a close message is receive a channel is considered to be valid to keep on sending replies to received messages.
Note that this situation can take some time because remote peer, at the moment of receiving a close message, could have tons of message waiting to be sent.
Vortex library consumer can use this function to avoid sending message over a channel that can be on an ongoing closing process.