Improve estimated StartTime calculation in Aggregation

[karianna]

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 * todo: The better way to do this is to calculate the variance in GC frequency and if the gap between 0.000 and
 * the first event exceeds the variance, then the first event would be considered the beginning of the log file.
 * otherwise, the time of the first event should be the time of the first event - the variance.

This removes the 0.25 magic number.