use a single TCP outgoing buffer for each client / session

[aler9]

this saves memory.

[codecov]

Codecov Report

Attention: Patch coverage is 82.08955% with 12 lines in your changes missing coverage. Please review.

Project coverage is 82.48%. Comparing base (6750427) to head (618fbde).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
client.go	73.91%	4 Missing and 2 partials ⚠️
server_session.go	75.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #665   +/-   ##
=======================================
  Coverage   82.47%   82.48%           
=======================================
  Files         116      116           
  Lines       11847    11851    +4     
=======================================
+ Hits         9771     9775    +4     
  Misses       1688     1688           
  Partials      388      388           

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