Use "async fn" for Service::ready() and Service::shutdown() methods

[fafhrd91]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 81.69399% with 67 lines in your changes are missing coverage. Please review.

Project coverage is 90.39%. Comparing base (d8f55de) to head (bb093e4).

Files	Patch %	Lines
ntex-service/src/pipeline.rs	80.31%	25 Missing ⚠️
ntex-service/src/boxed.rs	0.00%	17 Missing ⚠️
ntex-service/src/util.rs	77.41%	14 Missing ⚠️
ntex-service/src/lib.rs	27.27%	8 Missing ⚠️
ntex-service/src/map_config.rs	0.00%	2 Missing ⚠️
ntex-service/src/macros.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #361      +/-   ##
==========================================
- Coverage   90.87%   90.39%   -0.49%     
==========================================
  Files         197      198       +1     
  Lines       31754    31918     +164     
==========================================
- Hits        28858    28851       -7     
- Misses       2896     3067     +171     

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