use a single bytes.Reader for frame parsing

[marten-seemann]

Part of #3526. Depends on #3534.

[codecov]

Codecov Report

Base: 85.48% // Head: 85.48% // No change to project coverage 👍

Coverage data is based on head (0b26365) compared to base (0b26365).
Patch has no changes to coverable lines.

❗ Current head 0b26365 differs from pull request most recent head 1c37ff3. Consider uploading reports for the commit 1c37ff3 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3536   +/-   ##
=======================================
  Coverage   85.48%   85.48%           
=======================================
  Files         138      138           
  Lines       10226    10226           
=======================================
  Hits         8741     8741           
  Misses       1102     1102           
  Partials      383      383           

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