Close connection when _fill_free_pool is cancelled.

[iksteen]

Cancelling _fill_free_pool before a new connection is added to self._free prevents it from being cleaned up which causes a lingering connection and eventually the postgresql server running out of sockets.

What do these changes do?

Are there changes in behavior for the user?

Related issue number

Checklist

• [ ] I think the code is well written
• [ ] Unit tests for the changes exist
• [ ] Documentation reflects the changes
• [ ] Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • 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.

[codecov[bot]]

Codecov Report

Merging #893 (1f39c7c) into master (1ba66ca) will decrease coverage by 0.93%. The diff coverage is 20.00%.

@@            Coverage Diff             @@
##           master     #893      +/-   ##
==========================================
- Coverage   91.77%   90.83%   -0.94%     
==========================================
  Files           9        9              
  Lines         863      873      +10     
  Branches      118      121       +3     
==========================================
+ Hits          792      793       +1     
- Misses         53       61       +8     
- Partials       18       19       +1     

Impacted Files	Coverage Δ
aiopg/pool.py	91.02% <20.00%> (-3.45%)	:arrow_down:

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