Use floor() not rint() when reducing precision of fractional seconds in
timestamp_trunc, timestamptz_trunc, and interval_trunc(). This change
only affects the float-datetime case; the integer-datetime case already
behaved like truncation instead of rounding. Per gripe from Mario Splivalo.
This is a pre-existing issue but I'm choosing not to backpatch, because
it's such a corner case and there have not been prior complaints. The
issue is largely moot anyway given the trend towards integer datetimes.