Use kernel_id for new kernel if it doesn't exist in MappingKernelManager.start_kernel

[the-higgs]

Closes #403. This change aligns the implementation of MappingKernelManager.start_kernel with its docstring.

It is worth mentioning that in the API Doc, there is currently no method for starting a kernel by specifying a kernel_id. As such, no changes are made to jupyter_server/tests/services/kernels/test_api.py. Nevertheless, this change provides a possibility to support such functionality if that is required in future.

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[codecov-commenter]

Codecov Report

Merging #511 (3c7cf64) into master (858b6a7) will decrease coverage by 0.05%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master     #511      +/-   ##
==========================================
- Coverage   77.12%   77.07%   -0.06%     
==========================================
  Files         107      107              
  Lines        9490     9491       +1     
  Branches     1026     1027       +1     
==========================================
- Hits         7319     7315       -4     
- Misses       1804     1807       +3     
- Partials      367      369       +2     

Impacted Files	Coverage Δ
jupyter_server/services/kernels/kernelmanager.py	80.79% <25.00%> (+0.43%)	⬆️
jupyter_server/services/kernels/handlers.py	61.37% <0.00%> (-1.59%)	⬇️

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[Zsailer]

Thank you! Great stuff!

@kevin-bates or @vidartf, do you mind taking a second look?

[kevin-bates]

Yes, thank you @higgsb0. I just had one minor comment.

[kevin-bates]

Looks good - thank you!

[Zsailer]

Thanks, @higgsb0 and @kevin-bates!

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