Use async kernel manager by default

[kevin-bates]

Seems time we switched to using the async kernel management framework by default. This has already been adopted by NBclient (Papermill), Voila, and Enterprise Gateway.

[codecov-io]

Codecov Report

Merging #399 (9035592) into master (5013f62) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
- Coverage   68.87%   68.85%   -0.02%     
==========================================
  Files          57       57              
  Lines        6319     6319              
  Branches      846      846              
==========================================
- Hits         4352     4351       -1     
  Misses       1678     1678              
- Partials      289      290       +1     

Impacted Files	Coverage Δ
jupyter_server/serverapp.py	60.09% <ø> (ø)
jupyter_server/gateway/managers.py	77.58% <100.00%> (ø)
jupyter_server/services/kernels/handlers.py	58.24% <0.00%> (-0.27%)	⬇️

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