Pubsub receiver hang fix

[popravich]

Fixes #354.

• tests
• code
• docs

[codecov]

Codecov Report

Merging #366 into master will decrease coverage by 0.17%.
The diff coverage is 97.56%.

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
- Coverage   97.06%   96.88%   -0.18%     
==========================================
  Files          61       54       -7     
  Lines        9189     7414    -1775     
  Branches      780      539     -241     
==========================================
- Hits         8919     7183    -1736     
+ Misses        186      169      -17     
+ Partials       84       62      -22

Impacted Files	Coverage Δ
tests/pubsub_commands_test.py	100% <100%> (ø)	⬆️
aioredis/abc.py	100% <100%> (ø)	⬆️
tests/pubsub_receiver_test.py	98.81% <97.14%> (-0.31%)	⬇️
aioredis/commands/cluster.py	36.36% <0%> (-53.7%)	⬇️
tests/sentinel_failover_test.py	86.46% <0%> (-2.26%)	⬇️
aioredis/errors.py	100% <0%> (ø)	⬆️
aioredis/cluster/base.py
aioredis/cluster/testcluster.py
tests/cluster_test.py
aioredis/cluster/crc.py
... and 6 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8bd157e...62aead1. Read the comment docs.