Add Producer waitlist for pending send() items

[tvoinarovskyi]

Changes

Fixes #528

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]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.56%. Comparing base (ccbcc25) to head (0dcb78c).
⚠️ Report is 146 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #814      +/-   ##
==========================================
+ Coverage   97.49%   97.56%   +0.06%     
==========================================
  Files          30       30              
  Lines        5434     5452      +18     
==========================================
+ Hits         5298     5319      +21     
+ Misses        136      133       -3     

Flag	Coverage Δ
cext	88.40% <100.00%> (+0.09%)	⬆️
integration	97.52% <100.00%> (+0.06%)	⬆️
purepy	97.10% <100.00%> (+0.06%)	⬆️
unit	49.74% <88.23%> (+0.25%)	⬆️

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