Fix data race in TestWarmingReads

[GuptaManan100]

Description

This PR fixes the data race as pointed out in #14186.

The data race was happening on the replica tablet. In the test TestWarmingReads, warming read queries are run on the replica asynchronously. This lead to the Queries field in the sandboxconn being updated in a separate go-routine than the main go routine. This causes the read and writes to this variable in the main go routine to error with a data race.

This has been fixed in this PR by adding synchronization to that variable. This however has been added only when it is required so as to not hamper performance of the other tests.

The test TestWarmingReads was introduced in #13206

Related Issue(s)

• Fixes Data Race in TestWarmingReads #14186

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

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[shlomi-noach]

thank you