DateMath: Fix semantics of rounding with inclusive/exclusive ranges.

Date math rounding currently works by rounding the date up or down based
on the scope of the rounding.  For example, if you have the date
`2009-12-24||/d` it will round down to the inclusive lower end
`2009-12-24T00:00:00.000` and round up to the non-inclusive date
`2009-12-25T00:00:00.000`.

The range endpoint semantics work as follows:
* `gt` - round D down, and use > that value
* `gte` - round D down, and use >= that value
* `lt` - round D down, and use <
* `lte` - round D up, and use <=

There are 2 problems with these semantics:
* `lte` ends up including the upper value, which should be non-inclusive
* `gt` only excludes the beginning of the date, not the entire rounding scope

This change makes the range endpoint semantics symmetrical.  First, it
changes the parser to round up and down using the first (same as before)
and last (1 ms less than before) values of the rounding scope.  This
makes both rounded endpoints inclusive. The range endpoint semantics
are then as follows:
* `gt` - round D up, and use > that value
* `gte` - round D down, and use >= that value
* `lt` - round D down, and use < that value
* `lte` - round D up, and use <= that value

closes #8424