Allow services to handle many contests at the same time

At the moment many services require a contest ID as a command line argument. I think that their use of this ID is very limited (except in some cases like CWS). I think that they could manage multiple contest simultaneously. For example AWS already does that and, in fact, it doesn't require that command line argument. That would mean, for example, that Workers could accept job requests for different contests, that ES should manage submissions of many contests (all in the same queue?), and the same for SS.

I therefore suggest to check exactly how services use the contest ID and check if it's possible to remove it, making them able to handle data from all contests at the same time.

What do you think?

PS: the reason for this is that, for example, for training purposes it seems convenient to run many contest simultaneously on the same machine, and it seems excessive to require dedicated services for each of them (actually, I'm not even sure if, for example, many ESs on the same machine would work correctly...)