Add NVMe-oF connect concurrency limiter

[ashtonian]

Description

Same as before, it seems during mass reconnect events there can be contention around initializing the connection. This introduces a tunable to limit that.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Performance improvement
• Test addition or update

Motivation

Connecting 50+ volumes to a single host cascade and fails.

Changes Made

• Add concurrency limiter around nvmeof connections.

Testing

Deployed in my cluster, needs verification.

Test environment:

• Kubernetes version:
• TrueNAS version:
• Protocol tested: [ ] NFS [x] NVMe-oF [ ] Both

Tests run:

• Unit tests (make test)
• Linting (make lint)
• Sanity tests
• Integration tests (manual or CI)

Documentation

• Code comments updated
• README updated (if needed)
• Docs updated (if needed)
• No documentation needed

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Security Considerations

• This PR does not introduce security vulnerabilities
• No secrets or credentials are included in this PR
• I have reviewed SECURITY.md and followed best practices

Related Issues

Related to #130

Additional Notes

[fenio]

Integrated manually onto main as 49eb208 — thanks @ashtonian!