[Proposal] Include channel name in IOPub status messages

[Zsailer]

Another idea sparked by #838.

Today, both the control and shell channel can trigger IOPub status messages, but there is no easy way to tell which parent channel caused a given IOPub message. We could add "channel" as part of the IOPub's message specification, e.g.:

{
    ...
    "parent_header": {
        "channel": "shell",
        ...
    },
    "execution_state": ...
}

[minrk]

I like this solution. It should probably go in the content of the status message, since it's specifying what is idle/busy/etc.