Pub/Sub Receiver stop() fix

[popravich]

This fixes the case when pubsub receiver is blocked in get() and then stop() is called,
see test for details.

[codecov-io]

Codecov Report

Merging #211 into master will decrease coverage by 0.08%.
The diff coverage is 70.58%.

@@            Coverage Diff             @@
##           master     #211      +/-   ##
==========================================
- Coverage    95.6%   95.51%   -0.09%     
==========================================
  Files          56       56              
  Lines        7273     7286      +13     
  Branches      595      596       +1     
==========================================
+ Hits         6953     6959       +6     
- Misses        231      237       +6     
- Partials       89       90       +1

Impacted Files	Coverage Δ
tests/py35_pubsub_receiver_test.py	85.71% <70.58%> (-14.29%)	⬇️
aioredis/commands/transaction.py	84.72% <0%> (-1.39%)	⬇️
tests/sentinel_failover_test.py	86.46% <0%> (ø)	⬆️
aioredis/sentinel/pool.py	82.1% <0%> (ø)	⬆️

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