ILB: Optimize shared resource lock with fixed hashed pool and OCC

[08volt]

Replaced the heavy global mutex sharedResourceLock in the ILB controller with a hybrid locking strategy to eliminate serialization bottlenecks:

1. Fixed-Size Hashed Lock Pool (4096 buckets): Serializes creation/read-modify-write cycles for shared unmanaged resources (HealthChecks, InstanceGroups) without unbounded memory growth ($O(1)$ space complexity).
2. Optimistic Concurrency Control (OCC): Removes locks for BackendService updates, relying on native GCE API fingerprints to detect and retry conflicts.

This unblocks independent service reconciliations and prevents nodesync stalls during service update storms.

[k8s-ci-robot]

This issue is currently awaiting triage.

If the repository mantainers determine this is a relevant issue, they will accept it by applying the triage/accepted label and provide further guidance.

The triage/accepted label can be added by org members by writing /triage accepted in a comment.

Details

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: 08volt
Once this PR has been reviewed and has the lgtm label, please assign cheftako for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• providers/gce/OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment