feat: change how region id maps to region worker

[waynexia]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Change the static region worker mapping method. Remove hashing and use modulo instead.

Hashing mapping is not quite predictable. The new mapping considers table id and region number separately, so regions from one table won't get into the same worker.

This is a workaround and the workload skew problem still exists, and will exist if we don't support mechanisms like work stealing or dynamic mapping.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.30%. Comparing base (2035e7b) to head (25e80a2).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3384      +/-   ##
==========================================
- Coverage   85.38%   85.30%   -0.09%     
==========================================
  Files         891      893       +2     
  Lines      145544   147207    +1663     
==========================================
+ Hits       124276   125572    +1296     
- Misses      21268    21635     +367     

[fengjiachun]

LGTM