xpath OPTIMIZE try to first get_node_pos starting from previous found node
xpath OPTIMIZE try to first get_node_pos starting from previous found node

This takes advantage of sets being mostly ordered - huge impact
on performance.

lukasmacko/libyang,PavolVican/libyang,lukasmacko/libyang,PavolVican/libyang,PavolVican/libyang,PavolVican/libyang,PavolVican/libyang