Fix race condition when more than one contestant is typing

[thijzert]

Though the opportunity is minimal (save for a few for loops, no expensive operations are performed in any of the request handlers), since the quiz state is always saved as a whole, if two contestants are updating their submission concurrently, one will be ignored.

It's probably a good idea to separate each contestant's world in separate parts, and only save one at a time. Or at least employ separate 'dirty' flags for each one.