Make code and comment more clear

[yuki-uchida]

• delete unnecessary and comment
• add comment
• make processing order easier to understand
• distinguish between send and forward
  • send: server to client
  • forward: recv thread to write thread in server
• move moqt_client from core to server
• sort and group import

[codecov]

Codecov Report

Attention: Patch coverage is 47.44526% with 72 lines in your changes missing coverage. Please review.

Project coverage is 65.41%. Comparing base (f849b98) to head (5c2e54a).

Additional details and impacted files

@@             Coverage Diff              @@
##           draft-06     #130      +/-   ##
============================================
- Coverage     65.59%   65.41%   -0.18%     
============================================
  Files            62       63       +1     
  Lines          3104     3097       -7     
============================================
- Hits           2036     2026      -10     
- Misses         1068     1071       +3     

Components	Coverage Δ
moqt-core	91.19% <66.66%> (-0.36%)	⬇️
moqt-server	53.97% <42.72%> (+0.39%)	⬆️