Pool recylce

[jettify]

Based on similar feature from aiopg aio-libs/aiopg#373

[codecov]

Codecov Report

Merging #216 into master will decrease coverage by 0.18%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
- Coverage   91.78%   91.59%   -0.19%     
==========================================
  Files          12       12              
  Lines        1813     1821       +8     
  Branches      255      256       +1     
==========================================
+ Hits         1664     1668       +4     
- Misses         93       96       +3     
- Partials       56       57       +1

Impacted Files	Coverage Δ
aiomysql/sa/engine.py	100% <ø> (ø)	⬆️
aiomysql/pool.py	96.25% <62.5%> (-1.83%)	⬇️
aiomysql/connection.py	87.71% <75%> (-0.08%)	⬇️

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