Do not create additional goroutine for cleanup

[gammazero]

Clenaup can be done in the worker goroutine after the worker finishes running.

Also, avoid creating a new timer in each iteration of the worker run loop. Reuse the same timer instead.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (f53abbf) 73.78% compared to head (5360101) 73.82%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #395      +/-   ##
==========================================
+ Coverage   73.78%   73.82%   +0.03%     
==========================================
  Files         147      147              
  Lines        9603     9617      +14     
==========================================
+ Hits         7086     7100      +14     
+ Misses       1777     1774       -3     
- Partials      740      743       +3     

Files	Coverage Δ
service/datasetworker/datasetworker.go	80.38% <69.23%> (-1.18%)	⬇️

... and 7 files with indirect coverage changes

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[xinaxu]

Great lesson for me. I will need to use the timer in other places of this project too.