model: fixes for future value passing

model: fixes for future value passing

For one, we don't want to 'add_future_value()' when w_modification_order
is called anywhere besides process_write(). Also, we want to filter out
potential future values based on the existence of a Promise that this
write must resolve. So pass a vector parameter to w_modification_order
for recording future values only when (and where) we want them.