Make "wait" action interruptible

Assigned to Tender discussion #33.

This is almost a trivial change: Wait derives from Action, and Action derives from State, so you should be able to implement this by removing the sleep in Wait::execute and adding a Wait::next that returns an empty next state until the wait expires.

Unfortunately, TaskSystemState spoils the party by ignoring the inherent State-ness of Action and calling execute directly on each contained action. So it seems that, if you want to change Wait as described above, you also have to change TaskSystemState to behave more like Block or other paradigm components.