fix various race conditions when writing packets to closed clients or server sessions

[aler9]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 83.78378% with 18 lines in your changes missing coverage. Please review.

Project coverage is 82.00%. Comparing base (b2cfa93) to head (f706c65).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
client.go	78.43%	10 Missing and 1 partial ⚠️
server_session.go	85.41%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #684      +/-   ##
==========================================
+ Coverage   81.98%   82.00%   +0.01%     
==========================================
  Files         120      120              
  Lines       12414    12464      +50     
==========================================
+ Hits        10178    10221      +43     
- Misses       1821     1825       +4     
- Partials      415      418       +3     

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