setorder and x[order(.)] and integer64 support for descending order

[arunsrinivasan]

As the title suggests, integer64 support now only is capable of increasing order sort. To do: implement descending order as well. This is related to #603 (FR #2405), which is closed now.

[arunsrinivasan]

Commit d5b5134 addresses na.last=NA for integer64 (missed that in the earlier commit f4e9c95).