Nudge on the control channel instead of the shell

[JohanMabille]

Fixes #622.

Notice that we do not need to wait for this JEP to be accepted before merging this PR, since the current specification of the protocol does not prevent sending kernel_info requests on the control channel.

[codecov-commenter]

Codecov Report

Merging #628 (383dfae) into master (e5cb6c4) will decrease coverage by 0.06%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master     #628      +/-   ##
==========================================
- Coverage   77.63%   77.56%   -0.07%     
==========================================
  Files         110      110              
  Lines       10398    10393       -5     
  Branches     1408     1407       -1     
==========================================
- Hits         8072     8061      -11     
- Misses       1929     1933       +4     
- Partials      397      399       +2     

Impacted Files	Coverage Δ
jupyter_server/services/kernels/handlers.py	60.40% <71.42%> (-2.00%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e5cb6c4...383dfae. Read the comment docs.

[blink1073]

Nice, thank you!