Fix Redis stats storage concurrency issues

[yaronius]

Fixes #477

[codecov-io]

Codecov Report

Merging #478 into master will decrease coverage by <.01%.
The diff coverage is 89.65%.

@@            Coverage Diff             @@
##           master     #478      +/-   ##
==========================================
- Coverage   80.92%   80.91%   -0.01%     
==========================================
  Files          20       20              
  Lines        1431     1436       +5     
==========================================
+ Hits         1158     1162       +4     
- Misses        220      221       +1     
  Partials       53       53

Impacted Files	Coverage Δ
storage/redis/redis.go	100% <100%> (ø)	⬆️
gorush/notification_fcm.go	80.15% <83.33%> (+0.67%)	⬆️

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