Ensure curdoc lookups work in async context

[philippjfr]

• Ensures that state access in async contexts looks up correct curdoc
• Implement async support for onload (Support async callbacks inpn.state.onload  #3719)

[codecov]

Codecov Report

Merging #3776 (ac10f8d) into master (be3c59a) will increase coverage by 0.01%.
The diff coverage is 87.40%.

@@            Coverage Diff             @@
##           master    #3776      +/-   ##
==========================================
+ Coverage   83.41%   83.42%   +0.01%     
==========================================
  Files         214      214              
  Lines       31443    31559     +116     
==========================================
+ Hits        26227    26328     +101     
- Misses       5216     5231      +15     

Flag	Coverage Δ
ui-tests	33.74% <13.43%> (-0.10%)	⬇️
unitexamples-tests	76.38% <87.40%> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
panel/io/jupyter_server_extension.py	0.00% <0.00%> (ø)
panel/io/model.py	61.11% <50.00%> (-0.21%)	⬇️
panel/io/server.py	75.87% <80.76%> (+0.06%)	⬆️
panel/io/state.py	70.49% <86.84%> (+0.81%)	⬆️
panel/tests/test_server.py	99.78% <98.36%> (-0.22%)	⬇️
panel/io/document.py	66.08% <100.00%> (-0.30%)	⬇️
panel/tests/conftest.py	88.20% <100.00%> (+0.05%)	⬆️

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