[optimize](sink) Optimize the BE load balancing logic during concurrent imports.

[vinlee19]

Proposed changes

For example, consider multiple BEs in Apache Doris. The load is not balanced during concurrent imports. At the same checkpoint, multiple stream load tasks are directed to the same BE, causing server pressure on the backends because the Pos variables are the same within the checkpoint. To address this, we can calculate the destination BE using both the subTaskId and Pos variables.

Issue Number: close #xxx

Problem Summary:

Describe the overview of changes.

Checklist(Required)

1. Does it affect the original behavior: (Yes/No/I Don't know)
2. Has unit tests been added: (Yes/No/No Need)
3. Has document been added or modified: (Yes/No/No Need)
4. Does it need to update dependencies: (Yes/No)
5. Are there any changes that cannot be rolled back: (Yes/No)

Further comments

If this is a relatively large or complex change, kick off the discussion at [email protected] by explaining why you chose the solution you did and what alternatives you considered, etc...