fix: the dropping_regions guards should be dropped on procedure done

[MichaelScofield]

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What's changed and what's your intention?

The failed sqlness test shows that there might be some data race on dropping tables:

After digging into the codes, I think the problem is caused by the implementation that the procedure returned does not mean it's dropped. The logs suggest the same:

This PR fix it.

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.

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 85.20%. Comparing base (7ca0fa5) to head (9848cbc).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3771      +/-   ##
==========================================
- Coverage   85.51%   85.20%   -0.32%     
==========================================
  Files         946      946              
  Lines      159555   159566      +11     
==========================================
- Hits       136446   135953     -493     
- Misses      23109    23613     +504     

[fengjiachun]

LGTM