Minor optimization by avoiding fmt.Sprintf on hot path

[pior]

Context

While fmt.Sprintf is handy, and usually fast enough for most use-cases, we should probably avoid using it for string concatenation on the hot path.

Micro-benchmark:

BenchmarkBase/sprintf-8         	19425255	        61.85 ns/op	      32 B/op	       2 allocs/op
BenchmarkBase/concat-8          	84949484	        14.20 ns/op	       0 B/op	       0 allocs/op

Changes

• Replace fmt.Sprintf with string concatenation when building redis key names/prefixes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.31%. Comparing base (4f00f52) to head (2d6d6f4).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #962      +/-   ##
==========================================
- Coverage   67.13%   66.31%   -0.83%     
==========================================
  Files          29       29              
  Lines        4300     5729    +1429     
==========================================
+ Hits         2887     3799     +912     
- Misses       1135     1651     +516     
- Partials      278      279       +1     

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