threadsafe implementation (WIP)

[Jutho]

first commit: new implementation, still run original tests

[codecov-io]

Codecov Report

Merging #14 into master will increase coverage by 2.53%.
The diff coverage is 67.33%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   66.05%   68.58%   +2.53%     
==========================================
  Files           2        2              
  Lines         109      156      +47     
==========================================
+ Hits           72      107      +35     
- Misses         37       49      +12

Impacted Files	Coverage Δ
src/cyclicorderedset.jl	47.12% <47.12%> (ø)
src/LRUCache.jl	95.65% <95.23%> (+21.14%)	⬆️

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