[Cs3] mediator code updated

[Mikhail Nesterenko]

CS3 students,

Mediator code had a bug that resulted in occasional segfault: a
cleared flight was removing itself from the controller's waiting_ set
which invalidated the range-based-for of the broadcast().

I modified the code as follows. Now, instead of removing itself, a
flight goes into "cleared" status. After broadcast(), the controller
excutes an observe() function that launches a single cleared plane.

Let me know if this makes sense.

thanks,
--
Mikhail