Try memcached concurrency

[Dreamsorcerer]

Fixes #585.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.67%. Comparing base (f364562) to head (0385323).
⚠️ Report is 22 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1019      +/-   ##
==========================================
+ Coverage   99.58%   99.67%   +0.08%     
==========================================
  Files          32       32              
  Lines        3398     3398              
  Branches      115      115              
==========================================
+ Hits         3384     3387       +3     
+ Misses         12       10       -2     
+ Partials        2        1       -1     

Files with missing lines	Coverage Δ
tests/performance/test_concurrency.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[Dreamsorcerer]

Must have been resolved by the spawn fix.