fixes for date_part micro/millisecond precision

Hi all,

This patch fixes a problem where extract/date_part returns bad values
for microsecond and millisecond parts. Patch is attached. I'm not
sure if the solution is correct, but it cures this symptom.

before:
========
brent=# select date_part('milliseconds','2001-11-24 13:49:12.826833-05'::timestamp);
date_part
------------------
826.833000000001
(1 row)

brent=# select date_part('microseconds','2001-11-24 13:49:12.826833-05'::timestamp);
date_part
------------------
826833.000000001
(1 row)

after:
========
brent=# select date_part('milliseconds','2001-11-24 13:49:12.826833-05'::timestamp);
date_part
-----------
826.833
(1 row)

brent=# select date_part('microseconds','2001-11-24 13:49:12.826833-05'::timestamp);
date_part
-----------
826833
(1 row)

cheers.
brent

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