Ensure that Websocket messages are applied sequentially

[philippjfr]

Fixes #6603

Another attempt at this fix, this time I'm serializing the events into messages immediately.

[codecov]

Codecov Report

Attention: Patch coverage is 6.06061% with 31 lines in your changes missing coverage. Please review.

Project coverage is 72.47%. Comparing base (9404b43) to head (bcdd72a).

Files	Patch %	Lines
panel/io/document.py	6.06%	31 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (9404b43) and HEAD (bcdd72a). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (9404b43)	HEAD (bcdd72a)
	8	6

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7115      +/-   ##
==========================================
- Coverage   82.25%   72.47%   -9.79%     
==========================================
  Files         327      326       -1     
  Lines       48784    48764      -20     
==========================================
- Hits        40127    35341    -4786     
- Misses       8657    13423    +4766     

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