Handle tasks overrunning end of execution window

[hawesie]

All normal tasks have a end_time set which indicates when they should be completed by. The current state of the mdp executor means that during execution a task could run over its end time. Nothing cancels the task in this case. The code removed to fix #316 removed something that was supposed to do this but didn't really work. If we want this behaviour we should add it back in using a different mechanism.