`DT[order(.), ...]` could create index on DT

[jangorecki]

As we already have to compute expensive forder call we could cache its results as index where possible. This would fit nicely together with #3023
Of course not all cases of order should create index, but only those that order on DT columns, and having proper na.last alignment.

[jangorecki]

This issue could be automatically handled by #4386, more details on that in #3023.