Culling: ensure last_activity attr exists before use

[afshin]

Port of jupyter/notebook#5355

Also fixes a race condition in async kernel shutdown and fixes a couple of fixture names that needed a jp_ prefix.

[codecov-io]

Codecov Report

Merging #365 (29db7f9) into master (c2b2c31) will increase coverage by 0.26%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #365      +/-   ##
==========================================
+ Coverage   69.02%   69.29%   +0.26%     
==========================================
  Files          56       56              
  Lines        6134     6135       +1     
  Branches      804      804              
==========================================
+ Hits         4234     4251      +17     
+ Misses       1646     1616      -30     
- Partials      254      268      +14     

Impacted Files	Coverage Δ
jupyter_server/services/kernels/kernelmanager.py	80.23% <83.33%> (+16.74%)	⬆️
jupyter_server/__init__.py	61.53% <0.00%> (-38.47%)	⬇️
jupyter_server/terminal/__init__.py	80.00% <0.00%> (-12.00%)	⬇️
jupyter_server/utils.py	59.79% <0.00%> (-11.06%)	⬇️
jupyter_server/serverapp.py	61.26% <0.00%> (-0.75%)	⬇️
jupyter_server/base/handlers.py	67.49% <0.00%> (-0.46%)	⬇️
jupyter_server/services/contents/filemanager.py	67.02% <0.00%> (-0.22%)	⬇️
jupyter_server/services/contents/fileio.py	75.39% <0.00%> (+1.04%)	⬆️
...upyter_server/services/contents/filecheckpoints.py	63.15% <0.00%> (+1.75%)	⬆️
jupyter_server/services/contents/manager.py	88.32% <0.00%> (+2.39%)	⬆️
... and 1 more

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[Zsailer]

Thanks, @afshin!