connection recycling

[soar]

See aio-libs#373

[codecov-io]

Codecov Report

Merging #1 into master will increase coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
+ Coverage   93.39%   93.53%   +0.13%     
==========================================
  Files          23       23              
  Lines        3560     3572      +12     
  Branches      206      206              
==========================================
+ Hits         3325     3341      +16     
+ Misses        189      186       -3     
+ Partials       46       45       -1

Impacted Files	Coverage Δ
aiopg/sa/engine.py	100% <ø> (ø)	⬆️
tests/test_pool.py	99.75% <100%> (ø)	⬆️
aiopg/pool.py	98.91% <100%> (+1.6%)	⬆️
aiopg/sa/connection.py	91.46% <0%> (+0.31%)	⬆️

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