Add ability to warm up reused sessions

[philippjfr]

The --reuse-sessions functionality is a great way to speed up rendering of a session. However if an app has never been visited we still have to initialize the app first which means that until a server is fully warmed up serving apps can be slow. Especially when using --num-procs, i.e. when enabling multiple processes, it is quite difficult to fully warm up the server. Therefore we allow --warm to not simply run the applications but also pre-cache a session.

[codecov]

Codecov Report

Merging #4658 (3f25319) into main (ce4d940) will decrease coverage by 1.10%.
The diff coverage is 27.65%.

❗ Current head 3f25319 differs from pull request most recent head 9ec0a19. Consider uploading reports for the commit 9ec0a19 to get more accurate results

@@            Coverage Diff             @@
##             main    #4658      +/-   ##
==========================================
- Coverage   83.19%   82.10%   -1.10%     
==========================================
  Files         264      265       +1     
  Lines       37351    37375      +24     
==========================================
- Hits        31076    30685     -391     
- Misses       6275     6690     +415     

Flag	Coverage Δ
ui-tests	39.60% <10.63%> (-0.06%)	⬇️
unitexamples-tests	72.05% <27.65%> (-1.22%)	⬇️

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Impacted Files	Coverage Δ
panel/io/session.py	0.00% <0.00%> (ø)
panel/command/serve.py	16.66% <7.14%> (-19.29%)	⬇️
panel/io/document.py	46.32% <33.33%> (-6.82%)	⬇️
panel/io/server.py	71.45% <100.00%> (-5.18%)	⬇️
panel/io/state.py	70.84% <100.00%> (-1.89%)	⬇️
panel/tests/conftest.py	86.90% <100.00%> (-3.14%)	⬇️

... and 15 files with indirect coverage changes

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