Change valid from logic to accept already valid orders

[koeppelmann]

placeValidFromOrders is the default function to use if a user wants to place multiple orders in one tx.
However - this function requires to specify a valid from. The intention is to have the option to place an order that is only valid in the future.

However - often a user wants to place multiple orders that are valid immediately. The issue is that this function will reject an order if the valid from batch ID already passed. A simple improvement would be to instead of rejecting such an order to simply replace the "valid from" with the current batch ID. This would even potentially make a small gas costs improvement since users could just use 0 for valid from if they want their orders active immediately.