[Doc] on_startup & on_shutdown signals example with aiopg engine

[panagiks]

What do these changes do?

Add a documentation example usage of on_startup and on_shutdown signals by properly creating and disposing an aiopg connection engine.

Are there changes in behavior for the user?

No, just documentation.

Related issue number

Relates to #2131.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the changes folder
  • name it <issue_id>.<type> for example (588.bug)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[panagiks]

If you want me to add another example for redis just drop me a line and I'll add it in Wednesday.

[codecov-io]

Codecov Report

Merging #2196 into 2.2 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              2.2    #2196   +/-   ##
=======================================
  Coverage   97.06%   97.06%           
=======================================
  Files          38       38           
  Lines        7691     7691           
  Branches     1341     1341           
=======================================
  Hits         7465     7465           
  Misses        102      102           
  Partials      124      124

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[asvetlov]

Looks good but on_cleanup signal should be used for db shutdown.
on_shutdown is for graceful closing opened HTTP connections, e.g. closing web sockets and HTTP streaming responses, db connections most likely should be opened at this stage.
on_cleanup is for final cleanup, no web handlers are running at the moment of signal call.

[panagiks]

Should be good now :)

[asvetlov]

Thanks!

[lock]

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related bugs.

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