use native timestamps instead of time.Duration

[aler9]

this improves timestamp precision

[codecov]

Codecov Report

Attention: Patch coverage is 10.44776% with 60 lines in your changes missing coverage. Please review.

Project coverage is 81.53%. Comparing base (5ec470c) to head (35a32bc).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/rtptime/global_decoder2.go	0.00%	52 Missing ⚠️
client.go	20.00%	4 Missing ⚠️
server_session.go	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #627      +/-   ##
==========================================
- Coverage   81.98%   81.53%   -0.45%     
==========================================
  Files         115      116       +1     
  Lines       11615    11678      +63     
==========================================
  Hits         9522     9522              
- Misses       1693     1755      +62     
- Partials      400      401       +1     

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