Allow onload callbacks to be threaded

[philippjfr]

This allows pn.state.onload callbacks to take advantage of the threading support that was recently added.

[codecov]

Codecov Report

Merging #3081 (d31792e) into master (cf509cd) will increase coverage by 0.02%.
The diff coverage is 79.22%.

@@            Coverage Diff             @@
##           master    #3081      +/-   ##
==========================================
+ Coverage   83.42%   83.45%   +0.02%     
==========================================
  Files         190      190              
  Lines       25133    25177      +44     
==========================================
+ Hits        20968    21012      +44     
  Misses       4165     4165              

Impacted Files	Coverage Δ
panel/command/serve.py	39.26% <0.00%> (-0.42%)	⬇️
panel/config.py	62.04% <73.07%> (+1.48%)	⬆️
panel/io/state.py	71.13% <74.07%> (+2.65%)	⬆️
panel/io/server.py	73.75% <100.00%> (-0.34%)	⬇️
panel/tests/test_server.py	100.00% <100.00%> (ø)
panel/tests/conftest.py	94.85% <0.00%> (-1.48%)	⬇️
panel/widgets/select.py	94.40% <0.00%> (+0.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cf509cd...d31792e. Read the comment docs.