roles/dhcp: replace etcd locker with mutex

[BeryJu]

vibecoded btw

[netlify]

✅ Deploy Preview for beryju-gravity canceled.

Name	Link
🔨 Latest commit	c21e1e7
🔍 Latest deploy log	https://app.netlify.com/projects/beryju-gravity/deploys/69f7d405a124d100084006bc

[codecov]

Codecov Report

❌ Patch coverage is 57.89474% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.36%. Comparing base (0e51f2f) to head (c21e1e7).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
pkg/roles/dhcp/ipam_internal.go	57.89%	5 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2175      +/-   ##
==========================================
- Coverage   69.42%   69.36%   -0.07%     
==========================================
  Files         138      138              
  Lines        7585     7599      +14     
==========================================
+ Hits         5266     5271       +5     
- Misses       1786     1793       +7     
- Partials      533      535       +2     

Flag	Coverage Δ
bench	12.17% <57.89%> (+0.07%)	⬆️
e2e	45.29% <57.89%> (+<0.01%)	⬆️
unittest	55.38% <57.89%> (-0.04%)	⬇️

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