cltv
This function interprets the locktime for the given transaction, and returns the block height before which this tx cannot be published. By convention in bitcoin, depending of the value of locktime it might be a number of blocks or a number of seconds since epoch. This function does not support the case when the locktime is a number of seconds that is not way in the past. NB: We use this property in lightning to store data in this field.
Return
the block height before which this tx cannot be published.