fix(mock): add channel bound to avoid io bottleneck

[wilhelmagren]

The master collecting all worker buffers is bottleneck. This is because the workers produce buffers faster than the master can write them to disk. When mocking a very large file (>100 million rows) this bottleneck leads to program running out of system memory.

FIX:
add bounded channel, size is kind of arbitrary, and should depend on how much system memory is available.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (7753348) 20.67% compared to head (7aecfef) 20.67%.

Files	Patch %	Lines
src/mock.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #27   +/-   ##
=======================================
  Coverage   20.67%   20.67%           
=======================================
  Files           6        6           
  Lines         503      503           
=======================================
  Hits          104      104           
  Misses        399      399           

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