Add gokv.Store implementation for BigCache

[philippgille]

Closes #45.

[codecov-io]

Codecov Report

Merging #49 into develop will increase coverage by 0.28%.
The diff coverage is 88.23%.

@@             Coverage Diff             @@
##           develop      #49      +/-   ##
===========================================
+ Coverage    83.23%   83.51%   +0.28%     
===========================================
  Files           12       13       +1     
  Lines          853      904      +51     
===========================================
+ Hits           710      755      +45     
- Misses          85       90       +5     
- Partials        58       59       +1

Impacted Files	Coverage Δ
bigcache/bigcache.go	88.23% <88.23%> (ø)

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