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:

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