Refax: added some utils and simplifications to the receiver buffer

[ethouris]

This introduces new functions: cmpPos (to be used later) and packetAt which short cuts to reaching the packet at given position.

[codecov-commenter]

Codecov Report

Merging #2522 (0f72a76) into master (b76c8b2) will decrease coverage by 0.26%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master    #2522      +/-   ##
==========================================
- Coverage   66.00%   65.73%   -0.27%     
==========================================
  Files          99       99              
  Lines       19750    19751       +1     
==========================================
- Hits        13036    12984      -52     
- Misses       6714     6767      +53     

Impacted Files	Coverage Δ
srtcore/buffer_rcv.cpp	58.64% <59.61%> (-0.20%)	⬇️
srtcore/buffer_rcv.h	73.07% <66.66%> (+2.24%)	⬆️
srtcore/list.cpp	75.33% <0.00%> (-2.17%)	⬇️
srtcore/congctl.cpp	81.34% <0.00%> (-1.56%)	⬇️
srtcore/core.cpp	59.11% <0.00%> (-0.81%)	⬇️
srtcore/buffer_snd.cpp	67.10% <0.00%> (-0.66%)	⬇️
srtcore/queue.cpp	81.19% <0.00%> (-0.45%)	⬇️
srtcore/api.cpp	54.44% <0.00%> (-0.11%)	⬇️

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