Feature #237/use single connection

[argaen]

Closes #237

[codecov-io]

Codecov Report

Merging #257 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
+ Coverage   99.17%   99.18%   +<.01%     
==========================================
  Files          13       13              
  Lines         848      855       +7     
  Branches       88       88              
==========================================
+ Hits          841      848       +7     
  Misses          7        7

Impacted Files	Coverage Δ
aiocache/decorators.py	100% <100%> (ø)	⬆️

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