HistogramAggregator: Finer-grained rounding.

The way `HistogramAggregator` works is that for every value, it is going to
compute a rounded value, that basically looks like
`(value / interval) * interval` and use it as a key in a hash table to
aggregate counts.

However, the exact rounded value is not needed yet at that stage, all we need
is a value that uniquely identifies the bucket, such as `(value / interval)`.
We could only multiply with `interval` again when building the bucket: this way
the second step is only performed once per bucket instead of once per value.

Although this looks like a micro optimization for the case that was just
decribed, it makes more sense with the date rounding implementations that we
have that are more CPU-intensive.

Close #4800