VReplication Workflows (RowStreamer): explicitly set read only when creating snapshots in the copy phase

[rohit-nayak-ps]

Description

On a high write qps cluster, while copying a huge table, we noticed that the copy phase of a MoveTables workflow was significantly increasing replica lag. The workflow was streaming from replicas. Reducing the copy phase duration to 10 minutes did not help.

While auditing INNODB_TRX It was noticed that trx_is_read_only is set to 0. We expected it to be 1 since we do this

if _, err := conn.ExecuteFetch("set transaction isolation level repeatable read", 1, false); err != nil {
		return "", err
}
if _, err := conn.ExecuteFetch("start transaction with consistent snapshot", 1, false); err != nil {
		return "", err
}

This PR modifies the second query to explicitly set read only: like start transaction with consistent snapshot, read only. A unit test confirms that this works.

Related Issue(s)

#8056

Checklist

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• Documentation was added or is not required

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[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 68.39%. Comparing base (6cd09cc) to head (13bbd41).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...t/vttablet/tabletserver/vstreamer/snapshot_conn.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15690      +/-   ##
==========================================
+ Coverage   68.38%   68.39%   +0.01%     
==========================================
  Files        1556     1556              
  Lines      195120   195120              
==========================================
+ Hits       133427   133458      +31     
+ Misses      61693    61662      -31     

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