Fix creating sync subscription for the same method and path (#476)

[mthenw]

This PR fixes creating multiple sync subscription for the same path and method.

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: YES

[codecov]

Codecov Report

Merging #481 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #481      +/-   ##
==========================================
+ Coverage   70.73%   70.78%   +0.05%     
==========================================
  Files          35       35              
  Lines        2180     2184       +4     
==========================================
+ Hits         1542     1546       +4     
  Misses        574      574              
  Partials       64       64

Impacted Files	Coverage Δ
libkv/subscription.go	89.02% <100%> (+0.27%)	⬆️

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