Use async fn for Service::ready()

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 50 lines in your changes are missing coverage. Please review.

Project coverage is 89.28%. Comparing base (d8f55de) to head (4f24721). Report is 1 commits behind head on master.

:exclamation: Current head 4f24721 differs from pull request most recent head 0d46a59

Please upload reports for the commit 0d46a59 to get more accurate results.

Files	Patch %	Lines
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/pipeline.rs	96.66%	3 Missing :warning:
ntex-io/src/dispatcher.rs	77.77%	2 Missing :warning:
ntex-service/src/map_config.rs	0.00%	2 Missing :warning:
ntex-util/src/services/keepalive.rs	75.00%	2 Missing :warning:
ntex-service/src/macros.rs	90.90%	1 Missing :warning:
ntex-util/src/services/variant.rs	95.65%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #362 +/- ## ========================================== - Coverage 90.87% 89.28% -1.60% ========================================== Files 197 197 Lines 31754 31669 -85 ========================================== - Hits 28858 28276 -582 - Misses 2896 3393 +497 ```

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