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:

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

Checklist

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

Does this introduce a breaking change?

• Yes
• No

Further comments

No.