Fix InMemoryConnector handling of queueing_lock

[elemoine]

Closes #372

Successful PR Checklist:

• Tests
  • (not applicable?)
• Documentation
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• Had a good time contributing?

[codecov]

Codecov Report

Merging #374 (81674ac) into master (a22244c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #374   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           28        28           
  Lines         1602      1601    -1     
  Branches       173       173           
=========================================
- Hits          1602      1601    -1     

Impacted Files	Coverage Δ
procrastinate/testing.py	100.00% <100.00%> (ø)
procrastinate/cli.py	100.00% <0.00%> (ø)

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