dhcp: rework IPAM to not keep state in memory

[BeryJu]

No description provided.

[netlify]

✅ Deploy Preview for beryju-gravity canceled.

Name	Link
🔨 Latest commit	1ba0b68
🔍 Latest deploy log	https://app.netlify.com/projects/beryju-gravity/deploys/69590b45a192b10008c6430d

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.19%. Comparing base (0137c64) to head (1ba0b68).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
pkg/roles/dhcp/ipam_internal.go	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1999       +/-   ##
===========================================
+ Coverage   56.93%   70.19%   +13.26%     
===========================================
  Files         133      134        +1     
  Lines        7372     7369        -3     
===========================================
+ Hits         4197     5173      +976     
+ Misses       2718     1669     -1049     
- Partials      457      527       +70     

Flag	Coverage Δ
bench	12.39% <75.00%> (-0.25%)	⬇️
e2e	45.18% <75.00%> (?)
unittest	56.65% <37.50%> (-0.20%)	⬇️

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