Allow each Column in a DataTable have its own RowIndex

[st-pasha]

This is needed to implement efficient joins: the columns from the Left frame and the columns from the Right frame will have different RowIndices applied to them.
This feature may also be handy when selecting regular columns alongside with the computed ones.

This change will remove the rowindex property in DataTable object, possibly replacing it with nrowindices field counting the number of different rowindices for all columns in the frame.

[st-pasha]

Prerequisite: #1477