Defer Jupyter Comm initialization until frontend is ready

[philippjfr]

This resolves an issue when there is a race condition between the frontend being fully initialized (i.e. Bokeh and Panel JS and all extensions being loaded) and the backend trying to send an event to the frontend. This could occur if a user loaded the extension, rendered some component and then sent a message to it in rapid succession. Sending a message to the frontend would automatically attempt to open a Comm but if the frontend wasn't ready this Comm would never connect and we'd end up with a completely dead Comm, i.e. no messages could be sent across it.

This PR resolves this issue by waiting on the frontend to open the so called client comm, which indicates that the frontend is fully initialized. Only once the client comm is open we initialize the server comm. If a user attempts to send an event across the unopened server comm we schedule the message on the event loop until the comm has successfully been initialized.

Fixes #6178

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (c8f4e08) 82.68% compared to head (56ce133) 84.16%.
Report is 3 commits behind head on main.

Files	Patch %	Lines
panel/io/notebook.py	46.15%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6229      +/-   ##
==========================================
+ Coverage   82.68%   84.16%   +1.48%     
==========================================
  Files         301      301              
  Lines       45099    45110      +11     
==========================================
+ Hits        37288    37967     +679     
+ Misses       7811     7143     -668     

Flag	Coverage Δ
ui-tests	40.71% <30.76%> (+2.23%)	⬆️
unitexamples-tests	71.84% <46.15%> (+0.01%)	⬆️

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