Add get_event_loop_policy() to all get_event_loop() calls

[wbarnha]

Start using loop = asyncio.get_event_loop_policy().get_event_loop() since it may be safer as indicated by https://bugs.python.org/issue39529.

Plus we're using it in mode-streaming, so let's be consistent.

TO REVIEW AFTER MERGING #366.

[codecov-commenter]

Codecov Report

Base: 93.97% // Head: 93.88% // Decreases project coverage by -0.09% ⚠️

Coverage data is based on head (97624d4) compared to base (7bef2a2).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #415      +/-   ##
==========================================
- Coverage   93.97%   93.88%   -0.10%     
==========================================
  Files         102      102              
  Lines       11085    11085              
  Branches     1302     1516     +214     
==========================================
- Hits        10417    10407      -10     
- Misses        586      595       +9     
- Partials       82       83       +1     

Impacted Files	Coverage Δ
faust/streams.py	88.01% <ø> (ø)
faust/worker.py	95.38% <ø> (ø)
faust/cli/base.py	98.20% <100.00%> (ø)
faust/transport/base.py	100.00% <100.00%> (ø)
faust/transport/consumer.py	92.30% <100.00%> (-1.71%)	⬇️
faust/transport/drivers/aiokafka.py	73.49% <100.00%> (ø)
faust/transport/producer.py	89.16% <100.00%> (ø)

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