Refactor async io support

[fafhrd91]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 26 lines in your changes missing coverage. Please review.

Project coverage is 91.08%. Comparing base (db6d3a6) to head (15503f5).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
ntex-io/src/tasks.rs	87.91%	18 Missing ⚠️
ntex-io/src/testing.rs	91.66%	5 Missing ⚠️
ntex-io/src/buf.rs	75.00%	2 Missing ⚠️
ntex-io/src/filter.rs	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #417      +/-   ##
==========================================
+ Coverage   91.07%   91.08%   +0.01%     
==========================================
  Files         200      200              
  Lines       32320    32133     -187     
==========================================
- Hits        29436    29269     -167     
+ Misses       2884     2864      -20     

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