don't spawn a new Go routine to send a CONNECTION_REFUSED packet

[marten-seemann]

Depends on #4090.

[codecov]

Codecov Report

Merging #4091 (f766bf1) into master (9a397ab) will decrease coverage by 0.02%.
Report is 1 commits behind head on master.
The diff coverage is 55.56%.

❗ Current head f766bf1 differs from pull request most recent head eb99dd5. Consider uploading reports for the commit eb99dd5 to get more accurate results

@@            Coverage Diff             @@
##           master    #4091      +/-   ##
==========================================
- Coverage   83.74%   83.72%   -0.02%     
==========================================
  Files         150      150              
  Lines       15394    15378      -16     
==========================================
- Hits        12891    12875      -16     
- Misses       2003     2005       +2     
+ Partials      500      498       -2     

Files Changed	Coverage Δ
server.go	76.83% <55.56%> (+0.49%)	⬆️

... and 4 files with indirect coverage changes