Fixed process-pool for sync tasks.

[s3rius]

Closes: #611

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.37%. Comparing base (653400e) to head (b5ef21d).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
taskiq/receiver/receiver.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #616      +/-   ##
==========================================
- Coverage   79.41%   79.37%   -0.04%     
==========================================
  Files          69       69              
  Lines        2468     2473       +5     
==========================================
+ Hits         1960     1963       +3     
- Misses        508      510       +2     

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