Move buffer for RTP packets into internal

[Sean-Der]

Can be used by NACK and JitterBuffer now

[codecov]

Codecov Report

Attention: Patch coverage is 84.94624% with 14 lines in your changes missing coverage. Please review.

Project coverage is 72.76%. Comparing base (0eab188) to head (fa94c8c).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/rtpbuffer/rtpbuffer.go	82.00%	4 Missing and 5 partials ⚠️
internal/rtpbuffer/retainable_packet.go	90.00%	1 Missing and 1 partial ⚠️
pkg/nack/receive_log.go	33.33%	0 Missing and 2 partials ⚠️
internal/rtpbuffer/packet_factory.go	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
- Coverage   72.98%   72.76%   -0.22%     
==========================================
  Files          79       80       +1     
  Lines        3679     3679              
==========================================
- Hits         2685     2677       -8     
- Misses        863      867       +4     
- Partials      131      135       +4     

Flag	Coverage Δ
go	72.76% <84.94%> (-0.11%)	⬇️
wasm	70.37% <80.64%> (-0.63%)	⬇️

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