add support for redis

[guanw]

fix #13

[codecov]

Codecov Report

Merging #35 into master will decrease coverage by 1.39%.
The diff coverage is 82.5%.

@@           Coverage Diff            @@
##           master     #35     +/-   ##
========================================
- Coverage    93.3%   91.9%   -1.4%     
========================================
  Files          11      13      +2     
  Lines         269     309     +40     
========================================
+ Hits          251     284     +33     
- Misses         11      16      +5     
- Partials        7       9      +2

Impacted Files	Coverage Δ
plugins/storage/etcd/client.go	100% <ø> (ø)	⬆️
plugins/storage/factory.go	100% <100%> (ø)	⬆️
plugins/storage/redis/factory.go	50% <50%> (ø)
plugins/storage/redis/client.go	92.85% <92.85%> (ø)

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