Fix for multi-exec transaction cancelled error

[popravich]

This fixes the issue when using redis pool and executing 'slow' mutli-exec command:
pool-owned connection was getting released back to pool before result of exec was received.

[codecov-io]

Codecov Report

Merging #225 into master will increase coverage by 0.05%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
+ Coverage   96.67%   96.72%   +0.05%     
==========================================
  Files          55       55              
  Lines        7424     7425       +1     
  Branches      624      622       -2     
==========================================
+ Hits         7177     7182       +5     
+ Misses        187      183       -4     
  Partials       60       60

Impacted Files	Coverage Δ
tests/transaction_commands_test.py	100% <100%> (ø)	⬆️
aioredis/commands/transaction.py	86.95% <88.88%> (+2.23%)	⬆️
aioredis/sentinel/pool.py	90.23% <0%> (-0.53%)	⬇️
aioredis/commands/pubsub.py	96.2% <0%> (+0.94%)	⬆️

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