Limit generated timestamps to one second precision.

This commit changes all cases which generate default timestamps to
time.Now to limit the timestamp to one second precision.  The code which
serializes and deserializes timestamps already does this, but it is useful
to make sure defaults don't exceed the precision of the protocol either.

With this change there is less chance that developers using defaults will
end up with structures that have a higher time precision than what will
ultimately be sent across the wire.