Rename PodController to StatefulSetController

[enrichman]

This PR is a preparatory step for #454, intended to avoid making too many changes at once.

It renames the PodController to StatefulSetController, as its actual function was to reconcile the server's StatefulSet by watching the owned Pods. A minor refactor was also done without changing the core logic.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.99%. Comparing base (0bd681a) to head (09e77d2).

Files with missing lines	Patch %	Lines
main.go	33.33%	1 Missing and 1 partial ⚠️
pkg/controller/cluster/statefulset.go	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #482      +/-   ##
==========================================
+ Coverage   52.35%   52.99%   +0.64%     
==========================================
  Files          53       53              
  Lines        6460     6459       -1     
==========================================
+ Hits         3382     3423      +41     
+ Misses       2812     2775      -37     
+ Partials      266      261       -5     

Flag	Coverage Δ
cli	50.99% <60.00%> (+0.03%)	⬆️
controller	51.94% <73.33%> (+0.80%)	⬆️
e2e	51.94% <73.33%> (+0.80%)	⬆️
unit	41.28% <0.00%> (+<0.01%)	⬆️

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