Use certificate name as singleflight key

[johanbrandhorst]

Fixes #130

[codecov]

Codecov Report

Merging #131 (f65f387) into master (0b4cf6d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #131   +/-   ##
=======================================
  Coverage   63.98%   63.98%           
=======================================
  Files          10       10           
  Lines         547      547           
=======================================
  Hits          350      350           
  Misses        130      130           
  Partials       67       67           

Impacted Files	Coverage Δ
certify.go	63.63% <100.00%> (ø)

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