few fixes related to uvloop

[popravich]

• Fix "max clients" tests — uvloop closes connection earlier and so ConnectionClosedError is raised;
• Fix aioredis.StreamReader to buffer data until set_parser is called — uvloop reads and calls feed_data earlier than asyncio loop.

[codecov]

Codecov Report

Merging #347 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #347      +/-   ##
==========================================
+ Coverage   96.77%   96.83%   +0.06%     
==========================================
  Files          55       54       -1     
  Lines        7807     7309     -498     
  Branches      622      519     -103     
==========================================
- Hits         7555     7078     -477     
+ Misses        183      167      -16     
+ Partials       69       64       -5

Impacted Files	Coverage Δ
tests/conftest.py	88.57% <ø> (+0.12%)	⬆️
tests/pool_test.py	99.67% <100%> (ø)	⬆️
tests/connection_test.py	100% <100%> (ø)	⬆️
aioredis/stream.py	92.15% <100%> (+1.45%)	⬆️
aioredis/sentinel/pool.py	79.46% <0%> (-6.76%)	⬇️
tests/sentinel_failover_test.py	86.46% <0%> (-1.51%)	⬇️
aioredis/sentinel/__init__.py	100% <0%> (ø)	⬆️
examples/connection.py
aioredis/commands/pubsub.py	100% <0%> (+4.34%)	⬆️

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