vtworker: Minor fixes to percentage logging.
vtworker: Minor fixes to percentage logging.

- Do not use an abbrevation as variable name.
- Initialize the variable with 0.0 instead of relying on the zero value. This also looks nicer because we don't have the ugly "var" keyword in there :)
- Use float64 instead of float32 because that's the default type for float literals.
- Comment why we show 0% and not 100% for rowCount == 0.

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