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

[shashi]

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.

[jpsamaroo]

We should be able to wrap the call to compute in a try-catch, and also forward any exceptions that occur in async calls like async_apply and any try-catch blocks that already exist.

[jpsamaroo]

As @kshyatt pointed out, we'll also want the ability for thunks to cancel other thunks (supported by #117 ). e.g. optimization problems could use Dagger to schedule a number of concurrent "walker" thunks, and a "waiter" thunk will be co-executed and will wait until a walker thunk terminates with a suitable value below some threshold, and then terminate the rest of the walkers (and/or stop more from being scheduled).