http3: add http3.Server.ServeQUICConn to serve a single QUIC connection

[marten-seemann]

Recreating #3579, which was merged and which I had to force-revert from master, since there was a merge conflict after merging #3580.

[codecov]

Codecov Report

Base: 85.47% // Head: 85.47% // No change to project coverage 👍

Coverage data is based on head (c75bf49) compared to base (c75bf49).
Patch has no changes to coverable lines.

❗ Current head c75bf49 differs from pull request most recent head a25175f. Consider uploading reports for the commit a25175f to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3587   +/-   ##
=======================================
  Coverage   85.47%   85.47%           
=======================================
  Files         139      139           
  Lines       10245    10245           
=======================================
  Hits         8756     8756           
  Misses       1105     1105           
  Partials      384      384           

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