Cleanup sessions after warmup

[philippjfr]

Cleans up sessions after initializing them in panel serve command when --warm or --autoreload is provided.

[codecov]

Codecov Report

Merging #2601 (8e91400) into master (fb9eff9) will decrease coverage by 0.07%.
The diff coverage is 38.88%.

@@            Coverage Diff             @@
##           master    #2601      +/-   ##
==========================================
- Coverage   83.14%   83.07%   -0.08%     
==========================================
  Files         184      184              
  Lines       23264    23296      +32     
==========================================
+ Hits        19343    19353      +10     
- Misses       3921     3943      +22     

Impacted Files	Coverage Δ
panel/command/serve.py	14.63% <0.00%> (-1.88%)	⬇️
panel/io/server.py	61.33% <0.00%> (-0.30%)	⬇️
panel/io/callbacks.py	70.11% <75.00%> (+0.37%)	⬆️
panel/io/state.py	61.36% <83.33%> (+0.43%)	⬆️
panel/io/reload.py	66.29% <0.00%> (-2.25%)	⬇️

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