Add TAsyncProtocolBase and TAsyncTransportBase

[aiudirog]

As promised, I'm implementing proper interfaces and cleaning up some parts of the code. I'll let you know when this is ready.:)

[codecov[bot]]

Codecov Report

Merging #108 into master will decrease coverage by 0.25%. The diff coverage is 76.2%.

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
- Coverage   80.21%   79.95%   -0.26%     
==========================================
  Files          39       43       +4     
  Lines        3811     3887      +76     
==========================================
+ Hits         3057     3108      +51     
- Misses        754      779      +25

Impacted Files	Coverage Δ
thriftpy2/contrib/aio/protocol/binary.py	57.8% <100%> (+0.24%)	:arrow_up:
thriftpy2/transport/__init__.py	79.16% <100%> (-6.84%)	:arrow_down:
thriftpy2/contrib/aio/transport/framed.py	92.85% <100%> (ø)	:arrow_up:
thriftpy2/transport/framed/__init__.py	88.63% <100%> (ø)	:arrow_up:
thriftpy2/transport/buffered/__init__.py	95.12% <100%> (ø)	:arrow_up:
thriftpy2/contrib/aio/protocol/__init__.py	100% <100%> (ø)	:arrow_up:
thriftpy2/protocol/__init__.py	86.66% <100%> (+0.95%)	:arrow_up:
thriftpy2/transport/memory/__init__.py	88.23% <100%> (ø)	:arrow_up:
thriftpy2/contrib/aio/transport/buffered.py	94.73% <100%> (+0.73%)	:arrow_up:
thriftpy2/protocol/binary.py	90.31% <100%> (+0.03%)	:arrow_up:
... and 13 more

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[aiudirog]

@ethe I'm all set for now. Let me know what you think and if there is anything else you want to add.

[aiudirog]

I updated the docstring and included a link to our discussion. Since you approved the previous changes, I'll go ahead and merge it in. Thanks!