Added configurable parallelism pool size

[dmattia]

No description provided.

[codecov]

Codecov Report

Merging #1177 into master will increase coverage by 0.01%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #1177      +/-   ##
==========================================
+ Coverage   70.71%   70.72%   +0.01%     
==========================================
  Files          71       71              
  Lines        5975     5978       +3     
==========================================
+ Hits         4225     4228       +3     
  Misses       1398     1398              
  Partials      352      352              

Impacted Files	Coverage Δ
server/events/command_runner.go	53.92% <0.00%> (ø)
server/user_config.go	100.00% <ø> (ø)
cmd/server.go	79.75% <100.00%> (+0.16%)	⬆️
server/server.go	60.80% <100.00%> (+0.09%)	⬆️

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[dmattia]

On a PR with 412 plans being run in parallel, it takes 37 minutes without this change, and 6 minutes when I increase the parallelism limit on a repo of mine.

@antonbabenko I think this is ready for a review if you ever have a moment 😄