Improve applier .ReadMigrationRangeValues() func accuracy

[timvaillancourt]

Description

This PR moves the applier .ReadMigrationRangeValues() func to use a single transaction for both the min and max queries

Gathering these values in the same transaction is more accurate than the current approach where 2 x different auto-commit transactions grab these values

Lastly, the min/max funcs were made "private" because they're only called by .ReadMigrationRangeValues()

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[shlomi-noach]

This change is fine. Ultimately it does not matter that much: we take note of the binlog position before we evaluate the min/max range values. That means however we read the min/max values, any changes that happen in between -- we will address those changes as we iterate the binary logs; the changes will be covered by the binlog stream that we process.

[timvaillancourt]

This change is fine.

Thanks @shlomi-noach! Yeah I think so, I'm glad this didn't break any assumptions I didn't know about 👍

we will address those changes as we iterate the binary logs; the changes will be covered by the binlog stream that we process.

Agreed, I think this is more of a cosmetic change that won't actually affect the correctness of the end-result