Replace RwLock by OnceLock for transport callback

[wyfo]

Callback is set only once after the transport creation. Using the right primitive, e.g. a OnceLock gives better performance, as it just do an atomic load instead of a atomic RMWs of RwLock.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.47%. Comparing base (3c77c34) to head (219e18c).
⚠️ Report is 23 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2381      +/-   ##
==========================================
+ Coverage   71.91%   72.47%   +0.55%     
==========================================
  Files         390      390              
  Lines       62764    63283     +519     
==========================================
+ Hits        45139    45864     +725     
+ Misses      17625    17419     -206     

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