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

[fafhrd91]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 90.93%. Comparing base (dec6ab0) to head (b5afef6).
Report is 2 commits behind head on master.

Files	Patch %	Lines
ntex/src/http/h1/service.rs	46.15%	7 Missing ⚠️
ntex/src/http/service.rs	69.23%	4 Missing ⚠️
ntex/src/http/h2/service.rs	50.00%	2 Missing ⚠️
ntex/src/ws/client.rs	66.66%	2 Missing ⚠️
ntex/src/http/client/connector.rs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #364      +/-   ##
==========================================
+ Coverage   90.40%   90.93%   +0.53%     
==========================================
  Files         198      198              
  Lines       31918    31872      -46     
==========================================
+ Hits        28854    28984     +130     
+ Misses       3064     2888     -176     

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