* Allow flow property value persisters to rerurn a value to be set
after persistence operations. 
* If a non null value is returned by a persiter, set it back to flow
state.
* Considerations: 
 -- a property might be not accessed before flow gets to saveChanges
state. Thus it might be never set to flow states map.
 -- persistence happen very late in the flow, so persister supplied
value is more likely to be the desired one.