fix: improve node selection in sticky session

[zghong]

Description

Fixed the bug where chproxy always selects nodes only from a small subset of all nodes, for any random session_id.

Additionally, resolved performance bottlenecks caused by inefficient usage hash(sessionId) within loops.

Closes #574

Pull request type

Please check the type of change your PR introduces:

• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation content changes
• [ ] Other (please describe):

Checklist

• [x] Linter passes correctly
• [ ] Add tests which fail without the change (if possible)
• [x] All tests passing
• [ ] Extended the README / documentation, if necessary

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Further comments

No.