Fix event dispatch

[philippjfr]

There were some issues with bokeh events not correctly checking for an unblocked comm and therefore not dispatching events in a timely manner.

[codecov]

Codecov Report

Merging #3231 (f9758ea) into master (7d6044f) will decrease coverage by 0.00%.
The diff coverage is 92.30%.

@@            Coverage Diff             @@
##           master    #3231      +/-   ##
==========================================
- Coverage   83.10%   83.10%   -0.01%     
==========================================
  Files         193      193              
  Lines       25931    25930       -1     
==========================================
- Hits        21550    21549       -1     
  Misses       4381     4381              

Impacted Files	Coverage Δ
panel/reactive.py	77.17% <88.88%> (-0.03%)	⬇️
panel/io/state.py	69.80% <100.00%> (ø)
panel/tests/pane/test_image.py	100.00% <100.00%> (ø)

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