Modify MASP validity flag

[grarco]

Currently, we identify masp vp for the clients based on the fact that the masp vp was triggered and the tx was successful. This means that the MASP vp has to reject any transaction that triggers it but doesn't touch any masp keys because, if the tx also carried a valid Transaction object, this could trick clients into fetching and evaluating this tx.

We should add another condition to the previous two: the fact that at least one masp key was changed by the transaction. By doing this we could modify the MASP vp to accept txs that didn't touch any masp key and ensure that the clients' state won't be corrupted.