Cached supports stampede locking

[argaen]

Closes #248

Relates to #187

[codecov-io]

Codecov Report

Merging #249 into master will decrease coverage by 2.09%.
The diff coverage is 15%.

@@           Coverage Diff           @@
##           master   #249     +/-   ##
=======================================
- Coverage    99.1%    97%   -2.1%     
=======================================
  Files          13     13             
  Lines         784    802     +18     
  Branches       82     84      +2     
=======================================
+ Hits          777    778      +1     
- Misses          7     23     +16     
- Partials        0      1      +1

Impacted Files	Coverage Δ
aiocache/decorators.py	81.52% <10.52%> (-18.48%)	⬇️
aiocache/backends/memcached.py	100% <100%> (ø)	⬆️

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