internal/keyspan: refactor Truncate to not use filteringIter

[itsbilal]

Currently, keyspan.Truncate creates a filteringIter with a filter function. We uncovered a bug around certain uses of seeks in filteringIter in #3046 that forced us to add a check around Seek calls in the filteringIter, at the cost of one additional key comparison. If we refactored keyspan.Truncate to not use filteringIter, we could make that key comparison specific to just the Truncate case, and not other uses of filteringIter that don't mutate spans.

[RaduBerinde]

Fixed by #3282.