[core] Reduced calls to steady_clock::now() from two to one.

[maxsharabayko]

No description provided.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 65.96%. Comparing base (11701a6) to head (2af7b4d).
⚠️ Report is 343 commits behind head on master.

Files with missing lines	Patch %	Lines
srtcore/core.cpp	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2501      +/-   ##
==========================================
+ Coverage   65.57%   65.96%   +0.39%     
==========================================
  Files          99       99              
  Lines       19698    19698              
==========================================
+ Hits        12916    12993      +77     
+ Misses       6782     6705      -77     

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