Define web.Application.on_startup() signal handler

[f0t0n]

What do these changes do?

This change introduces new signal Application.on_startup()
that allows to register as many on_startup signal handlers as needed to run in the event loop along with the application's request handler just after the application's startup.
This is useful to run background tasks such as listening to different event sources (message queues, etc.) to process within an application (e.g. to forward to clients connected via WebSocket).

Are there changes in behavior for the user?

The change doesn't impact on existing behavior but extends the API and the functionality.

Related issue number

#1092

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

---

Changes done

• Define web.Application.on_startup() signal handler.
• Run app.on_startup() along with the request handler within an event
  loop in web.run_app().
• Tests for Application.on_startup signal
• Tests for multiple tasks to run on startup, including two long running
• Extend tests on web.run_app(): ensure that Application.startup()
  is called within web.run_app().
• Add documentation for Application.on_startup signal handler:
  • Describe possible use cases
  • Minor: fix typo

Notes

In the tests/test_run_app.py increased delay for loop.call_later from
0.01s to 0.02s since with the old value loop used to stop
before coroutines gathered for on_startup finished, that caused
an exception.

As discussed earlier, the appropriate changes in the aiohttp.worker.GunicornWebWorker will be applied in a separate PR.

[codecov-io]

Current coverage is 97.63% (diff: 100%)

Merging #1103 into master will increase coverage by 1.41%

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[asvetlov]

Thanks you very much!

[f0t0n]

Thank you for your help! :)

[lock]

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