Replace the dispatch semaphore into os_unfair_lock, copied from SDWebImage Core

[dreampiggy]

See more: SDWebImage/SDWebImage#3128

[codecov]

Codecov Report

Merging #51 (ba51bef) into master (049cc75) will decrease coverage by 1.85%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
- Coverage   77.25%   75.39%   -1.86%     
==========================================
  Files           2        2              
  Lines         809      813       +4     
==========================================
- Hits          625      613      -12     
- Misses        184      200      +16     

Flag	Coverage Δ
ios	75.39% <100.00%> (-1.86%)	⬇️

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Impacted Files	Coverage Δ
SDWebImageWebPCoder/Classes/SDImageWebPCoder.m	75.27% <100.00%> (-1.87%)	⬇️

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