Add _lock to BlockingConnectionPool

[abrookins]

What do these changes do?

Fixes #1047.

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
• 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> (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]

Codecov Report

Merging #1068 (3e9d710) into master (f9b0590) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1068      +/-   ##
==========================================
+ Coverage   89.18%   89.19%   +0.01%     
==========================================
  Files          21       21              
  Lines        6816     6823       +7     
  Branches      653      653              
==========================================
+ Hits         6079     6086       +7     
+ Misses        570      567       -3     
- Partials      167      170       +3     

Flag	Coverage Δ
unit	89.16% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aioredis/connection.py	66.01% <100.00%> (+0.24%)	⬆️
tests/test_connection_pool.py	99.30% <100.00%> (+0.01%)	⬆️
aioredis/client.py	82.20% <0.00%> (-0.11%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f9b0590...3e9d710. Read the comment docs.

[Andrew-Chen-Wang]

lgtm