Patch: Avoid precision error in to_timestamp().

Hello hackers,

I stumbled upon a precision issue with the to_timestamp() function that
causes it to return unexpected timestamp values. For instance, the query
SELECT to_timestamp(1486480176.236538) returns the timestamp "2017-02-07
16:09:36.236537+01", which is off by one microsecond. Looking at the source
code, the issue seems to be that the conversion is unnecessarily done using
imprecise floating point calculations. Since the target timestamp has
microsecond precision, and is internally represented by a 64-bit integer
(on modern platforms), it is better to first convert the given floating
point value to a microsecond integer and then doing the epoch conversion,
rather than doing the conversion using floating point and finally casting
to an integer/timestamp.

I am attaching a patch that fixes the above issue.

Regards,

Erik