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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