fix(fill): correct type for fillValueTime

[scbrickley]

Closes #5127

fill() was incorrectly setting fillValueTime as an int64, which caused a panic the next time fillTimeColumn was called and tried to coerce fillValueTime into a values.Time. This only manifested when processing a table that spanned multiple table chunks (i.e., when fillTimeColumn was called with a fillValue that was already set).

This patch sets fillValueTime to be a values.Time, which avoids the bad type coercion and thereby also avoids the panic.

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