Refactor worker management

[fafhrd91]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 85.27363% with 148 lines in your changes are missing coverage. Please review.

Project coverage is 91.14%. Comparing base (ca16e4a) to head (5b18a64).

Files	Patch %	Lines
ntex-server/src/manager.rs	82.11%	39 Missing ⚠️
ntex-server/src/wrk.rs	84.21%	30 Missing ⚠️
ntex/src/server/service.rs	86.13%	14 Missing ⚠️
ntex-server/src/server.rs	79.36%	13 Missing ⚠️
ntex/src/server/accept.rs	80.32%	12 Missing ⚠️
ntex-server/src/signals.rs	58.33%	10 Missing ⚠️
ntex/src/server/factory.rs	92.47%	7 Missing ⚠️
ntex-server/src/lib.rs	55.55%	4 Missing ⚠️
ntex/src/server/config.rs	96.52%	4 Missing ⚠️
ntex-server/src/pool.rs	90.32%	3 Missing ⚠️
... and 7 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #312      +/-   ##
==========================================
+ Coverage   90.69%   91.14%   +0.44%     
==========================================
  Files         191      197       +6     
  Lines       31556    31540      -16     
==========================================
+ Hits        28619    28746     +127     
+ Misses       2937     2794     -143     

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