Memory driver improvements

[flimzy]

No description provided.

[codecov]

Codecov Report

Merging #141 into master will increase coverage by 1.96%.
The diff coverage is 98.4%.

@@            Coverage Diff             @@
##           master     #141      +/-   ##
==========================================
+ Coverage   73.31%   75.28%   +1.96%     
==========================================
  Files          68       69       +1     
  Lines        2563     2674     +111     
==========================================
+ Hits         1879     2013     +134     
+ Misses        531      506      -25     
- Partials      153      155       +2

Impacted Files	Coverage Δ
driver/memory/store.go	100% <100%> (+100%)	⬆️
driver/memory/memory.go	100% <100%> (ø)	⬆️
driver/memory/rand.go	100% <100%> (ø)
driver/memory/db.go	61.64% <95.45%> (+55.76%)	⬆️
db.go	73.77% <0%> (+4.91%)	⬆️
driver/common/common.go	77.77% <0%> (+77.77%)	⬆️

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