Applyschema uses ExecuteMultiFetchAsDba

[shlomi-noach]

Description

See context in #15505

In this PR ApplySchema uses ExecuteMultiFetchAsDba() instead of ExecuteFetchAsDba(). This is scheduled for v22. In v23 we will be able to clean up ExecuteFetchAsDba() as ApplySchema is the last use of ExecuteFetchAsDba() that is still allowed to pass multiple statements.

Note: hardly any tests changed, because the point is that the behavior is unchanged and that all the tests continue as expected when we switch from ExecuteFetchAsDba() to ExecuteMultiFetchAsDba().

Related Issue(s)

#15505

Checklist

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• Documentation was added or is not required

Deployment Notes

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[codecov]

Codecov Report

Attention: Patch coverage is 53.33333% with 7 lines in your changes missing coverage. Please review.

Project coverage is 67.15%. Comparing base (4e385ce) to head (f34537d).

Files with missing lines	Patch %	Lines
go/vt/schemamanager/local_controller.go	0.00%	4 Missing ⚠️
go/vt/schemamanager/tablet_executor.go	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17078      +/-   ##
==========================================
- Coverage   67.16%   67.15%   -0.01%     
==========================================
  Files        1571     1571              
  Lines      252097   252101       +4     
==========================================
- Hits       169318   169303      -15     
- Misses      82779    82798      +19     

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