refact cmd/crowdsec: encapsulate cache into alertBuffer

[mmetc]

simplify loop and reduce risk of races (we didn't lock before reading len)

[github-actions]

@mmetc: There are no area labels on this PR. You can add as many areas as you see fit.

• /area agent
• /area local-api
• /area cscli
• /area appsec
• /area security
• /area configuration

Details

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[codecov]

Codecov Report

❌ Patch coverage is 69.04762% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.26%. Comparing base (a3dbe7f) to head (a46687c).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/crowdsec/output.go	62.06%	9 Missing and 2 partials ⚠️
cmd/crowdsec/serve.go	0.00%	0 Missing and 1 partial ⚠️
cmd/crowdsec/win_service.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4300      +/-   ##
==========================================
- Coverage   63.28%   63.26%   -0.03%     
==========================================
  Files         472      472              
  Lines       33382    33384       +2     
==========================================
- Hits        21126    21120       -6     
- Misses      10142    10151       +9     
+ Partials     2114     2113       -1     

Flag	Coverage Δ
bats	46.71% <70.73%> (+0.01%)	⬆️
unit-linux	36.13% <0.00%> (+0.07%)	⬆️
unit-windows	24.46% <0.00%> (-0.05%)	⬇️

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