Jupyter messaging updates/correctness

[halleysfifthinc]

• Bump Julia compat to be able to use pkgversion in "kernel_info_request" to report the IJulia version
• Some messages and responses were sent on the wrong channel:
  • Replies should be sent to the channel the request was sent on
    • This is not 💯 explicit in the messaging protocol docs, but appears to be the norm. Some messages are only described as always being sent (by the frontend/client) on a specific channel, but e.g. I saw "kernel_info_request" sent on both the shell/requests channel AND the control channel. (The latter is not described as possible according to the docs.)
• Replies should always have a { 'status': 'ok' }
• The transient metadata field was added in protocol version 5.1. Without the ability to add transient metadata, "update_display_data" messages can't be used to update/redisplay existing outputs (while participating in/with the rest of the IJulia display machinery). Related PR Function to publish raw/encoded display data #946 is more extensive and stalled; this is the minimum necessary change to support this.

Fixes #953
Fixes #1000

Reference: https://jupyter-client.readthedocs.io/en/latest/messaging.html