Fix possible race in MySQL startup and vttablet in parallel

[GuptaManan100]

Description

This PR fixes the race as described in #15537.
We now wait for the dba grants to have applied before we run any query that creates a connection with the said users.

Related Issue(s)

• Fixes Bug Report: Race possible in MySQL startup and vttablet in parallel #15537

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 65.72%. Comparing base (696fe0e) to head (56d7976).
Report is 138 commits behind head on main.

Files	Patch %	Lines
go/vt/mysqlctl/backup.go	0.00%	6 Missing ⚠️
go/vt/mysqlctl/fakemysqldaemon.go	0.00%	2 Missing ⚠️
go/vt/mysqlctl/mysqld.go	90.90%	2 Missing ⚠️
go/vt/vttablet/tabletmanager/tm_init.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15538      +/-   ##
==========================================
- Coverage   67.41%   65.72%   -1.70%     
==========================================
  Files        1560     1560              
  Lines      192752   194581    +1829     
==========================================
- Hits       129952   127891    -2061     
- Misses      62800    66690    +3890     

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[GrahamCampbell]

Can this be backported to v19?