Improved document cleanup when not invoked using server_destroy

[philippjfr]

As the title says, improves logic for cleaning up Document sessions created programmatically.

[codecov]

Codecov Report

Merging #3842 (b8fca79) into master (23b061b) will decrease coverage by 7.57%.
The diff coverage is 52.63%.

@@            Coverage Diff             @@
##           master    #3842      +/-   ##
==========================================
- Coverage   82.86%   75.29%   -7.58%     
==========================================
  Files         222      222              
  Lines       32426    32451      +25     
==========================================
- Hits        26871    24433    -2438     
- Misses       5555     8018    +2463     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	75.29% <52.63%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
panel/io/document.py	55.97% <23.80%> (-4.90%)	⬇️
panel/io/convert.py	29.43% <50.00%> (+0.80%)	⬆️
panel/io/server.py	75.24% <66.66%> (+0.29%)	⬆️
panel/config.py	60.43% <100.00%> (ø)
panel/io/state.py	69.35% <100.00%> (+0.19%)	⬆️
panel/reactive.py	77.44% <100.00%> (-0.70%)	⬇️
panel/template/base.py	76.16% <100.00%> (+0.17%)	⬆️
panel/tests/conftest.py	80.34% <100.00%> (-7.76%)	⬇️
panel/tests/ui/widgets/test_tabulator.py	9.79% <0.00%> (-89.10%)	⬇️
panel/tests/ui/widgets/test_input.py	11.33% <0.00%> (-88.09%)	⬇️
... and 25 more

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