WIP [P/D] Use ThreadPoolExecutor to do handshake for each P-D pair

[lk-chen]

Essential Elements of an Effective PR Description Checklist

• [x] The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• [x] The test plan, such as providing test command.
• [ ] The test results, such as pasting the results comparison before and after, or e2e results
• [ ] (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

Split from #19447, this PR keeps using zmq for nixl metadata transfer, but uses ThreadPoolExecutor to do _nixl_handshake in background.

closes #19777

Test Plan

Unit test WIP

Test Result

(Optional) Documentation Update

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[mergify[bot]]

This pull request has merge conflicts that must be resolved before it can be merged. Please rebase the PR, @lk-chen.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[lk-chen]

close in favor of #19836