ctrl+c while in the scheduler should raise SIGINT on all processes

We may have to wrap this whole while loop in a try catch:
https://github.com/JuliaParallel/Dagger.jl/blob/master/src/scheduler.jl#L108

and watch for InterruptExceptions, which then get forwarded to the worker procs.

I'm not sure how this works out when the workers are running many threads.