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

[codecov[bot]]

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 :warning:
ntex-service/src/boxed.rs	0.00%	17 Missing :warning:
ntex-service/src/util.rs	77.41%	14 Missing :warning:
ntex-service/src/lib.rs	27.27%	8 Missing :warning:
ntex-service/src/map_config.rs	0.00%	2 Missing :warning:
ntex-service/src/macros.rs	85.71%	1 Missing :warning:

Additional details and impacted files

```diff @@ 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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