skippable missing schema alerts and initialize time and fix HIST on nans

change 1:
Move missing schema alert from engine to client. 
Having the alert in engine makes it unskippable, which disrupts the 
utility of setting yes=True like on every other command that asks for 
confirmation. Moving it to client makes it so that yes=True bypasses
the alert.

change 2:
model['time'] is not always initialized in the persistence layer before 
adding to it.