Investigate the implications of the client choosing whether to sample from row or column

[musalbas]

In the fraud proofs paper, the client picks the (x, y) co-ordinates, but the node decides whether to return a response from the row or column root.

In the current implementation, the client also decides whether the response should be from a row or column root.

We should consider the security implications to this if, any.

[adlerjohn]

Also related: https://github.com/lazyledger/lazyledger-specs/issues/110#issuecomment-818786351