VTTablet schema reload: include stats only on periodic reload for all flavors. Default to "base table" query for the "with sizes" query for 5.7 only

[rohit-nayak-ps]

Description

For a specific MoveTables workflow, we are seeing timeouts on reloading the schema on unmanaged tables connected to the multi-tenant Aurora cluster with a massive number of tables.

For schema reloads, we are defaulting to using the "with sizes" query instead of just the "base tables" one. The "with sizes" one is significantly slower and is only needed by Online DDL migrations which we don't do on the unmanaged keyspaces.

This PR changes the VReplication related schema reloads to not include file size stats. We do reload stats for the periodic reload that is done on a timer at interval defined by the vttablet flag --queryserver-config-schema-reload-time.

Vitess now only supports MySQL 5.7 for imports, since 5.7 is EOL. So for the 5.7 flavor, which is the one used for many of the Aurora and RDS imports, we will just default to the "base tables" query instead of the "with sizes" one even if include_stats is specified to schema reload.

Related Issue(s)

Fixes #17856

Checklist

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Deployment Notes

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[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 67.47%. Comparing base (413b953) to head (ec98a9d).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
go/mysql/flavor_mysql.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17855      +/-   ##
==========================================
+ Coverage   67.44%   67.47%   +0.02%     
==========================================
  Files        1592     1593       +1     
  Lines      258205   258891     +686     
==========================================
+ Hits       174151   174681     +530     
- Misses      84054    84210     +156     

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[shlomi-noach]