Add cmux.ErrServerClosed

[albertteoh]

Signed-off-by: albertteoh albert.teoh@logz.io

Which problem is this PR solving?

• Fixes Flaky test TestServerGracefulExit #3025

Short description of the changes

• Add expected cmux.ErrServerClosed error when connection is closed when waiting to Accept.

[yurishkuro]

nice catch

[codecov]

Codecov Report

Merging #3029 (fd3cf89) into master (f0d0518) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3029      +/-   ##
==========================================
+ Coverage   95.94%   95.96%   +0.02%     
==========================================
  Files         224      224              
  Lines        9773     9773              
==========================================
+ Hits         9377     9379       +2     
+ Misses        326      325       -1     
+ Partials       70       69       -1     

Impacted Files	Coverage Δ
cmd/query/app/server.go	97.08% <100.00%> (-1.46%)	⬇️
plugin/storage/integration/integration.go	77.90% <0.00%> (+0.55%)	⬆️
cmd/query/app/static_handler.go	96.77% <0.00%> (+2.41%)	⬆️

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