fix: avoid blocking SMD cache during update

[alexlovelltroy]

Description

This pull request refactors the PopulateNodes method in SMDclient.go to improve thread safety and atomicity of updates, ensuring that cached lookups and modifications are not blocked by slow network operations. Additionally, a new performance test is added to verify that cache operations remain non-blocking during slow refreshes.

Thread safety and atomicity improvements:

• Refactored PopulateNodes to build new maps (nextNodes, nextIPToXname, etc.) off-lock, and only acquire the write lock briefly to atomically swap in the new state, preventing cache lookups from being blocked by network I/O. [1] [2]

Testing enhancements:

• Added a new test, TestPopulateNodesBlockedRefreshDoesNotBlockCachedOperations, to ensure that cached methods like IDfromIP and AddWGIP remain responsive even when PopulateNodes is blocked on a network call.

Checklist

• My code follows the style guidelines of this project
• I have added/updated comments where needed
• I have added tests that prove my fix is effective or my feature works
• I have run make test (or equivalent) locally and all tests pass
• I have updated the relevant documentation (CLI examples, man pages, README, other docs, etc.)
• DCO Sign-off: All commits are signed off (git commit -s) with my real name and email
• REUSE Compliance:
  • Each new/modified source file has SPDX copyright and license headers
  • Any non-commentable files include a <filename>.license sidecar
  • All referenced licenses are present in the LICENSES/ directory

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Dependency update

---

For more info, see Contributing Guidelines.

[travisbcotton]

Looks good to me