Added max-tasks-per-child parameter.

[s3rius]

Closes #309.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 55.55556% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 77.88%. Comparing base (ed37be4) to head (054383b).

Files	Patch %	Lines
taskiq/receiver/receiver.py	55.55%	4 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #314      +/-   ##
===========================================
- Coverage    78.00%   77.88%   -0.12%     
===========================================
  Files           61       61              
  Lines         1791     1800       +9     
===========================================
+ Hits          1397     1402       +5     
- Misses         394      398       +4     

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