Wrong emulation of EXTRACT() with DECADE, CENTURY, MILLENIUM date parts

[lukaseder]

In some dialects, EXTRACT() may yield non-integer results. For example:

-- Input
extract(millennium from date '2001-01-01')

-- Output in CockroachDB
(extract(year from date '2001-01-01') / 10)

When executed, this yields 200.1, not 200, due to the fact that that EXTRACT() produces a floating point number.

[lukaseder]

A lot of features depend on EXTRACT(), which means a backport will bring with it some slight regression risk. I won't backport this fix.

[lukaseder]

It is needed for #7900, though.