Ensure pn.state.execute dispatches immediately if possible

[philippjfr]

The logic was wrong and pn.state.execute was scheduling stuff on the event loop when we knew the document to be unblocked.

Fixes #3855

[codecov]

Codecov Report

Merging #3859 (3eda3aa) into master (1a2be00) will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #3859      +/-   ##
==========================================
- Coverage   82.94%   82.93%   -0.01%     
==========================================
  Files         224      224              
  Lines       32763    32763              
==========================================
- Hits        27174    27172       -2     
- Misses       5589     5591       +2     

Flag	Coverage Δ
ui-tests	34.40% <0.00%> (+<0.01%)	⬆️
unitexamples-tests	75.17% <75.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
panel/widgets/tables.py	88.31% <66.66%> (ø)
panel/io/state.py	69.35% <100.00%> (ø)
panel/util.py	87.08% <0.00%> (-0.37%)	⬇️
panel/tests/ui/widgets/test_tabulator.py	98.77% <0.00%> (-0.17%)	⬇️
panel/io/server.py	74.85% <0.00%> (+0.19%)	⬆️
panel/tests/util.py	88.34% <0.00%> (+0.97%)	⬆️

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