Distinct error for futures cancellation

[popravich]

Cancel pending futures with one of distinct errors (ConnectionClosedError,
ConnectionForcedCloseError or builtin exception like ConnectionError)
instead of calling cancel() method.

[codecov]

Codecov Report

Merging #316 into master will decrease coverage by 0.08%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #316      +/-   ##
==========================================
- Coverage    96.9%   96.82%   -0.09%     
==========================================
  Files          60       60              
  Lines        7659     7679      +20     
  Branches      609      611       +2     
==========================================
+ Hits         7422     7435      +13     
- Misses        175      179       +4     
- Partials       62       65       +3

Impacted Files	Coverage Δ
tests/integrational_test.py	96.61% <ø> (ø)	⬆️
tests/transaction_commands_test.py	100% <100%> (ø)	⬆️
aioredis/errors.py	100% <100%> (ø)	⬆️
aioredis/commands/transaction.py	83.33% <50%> (-3.63%)	⬇️
tests/connection_commands_test.py	94.44% <66.66%> (-1.64%)	⬇️
examples/py34/connection.py	92.03% <0%> (-0.59%)	⬇️
tests/conftest.py	89.97% <0%> (-0.51%)	⬇️
tests/sentinel_failover_test.py	87.96% <0%> (+0.75%)	⬆️
aioredis/util.py	92% <0%> (+3%)	⬆️

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[popravich]

This would resolve #306 and #190