Round-Robin Scheduling of Ready Tasks

[insertinterestingnamehere]

@hfingler noted in the N-body demo that letting the scheduler pick which GPU to use for each task instead of specifying them manually was actually harmful in that case. Our current best-guess for why that would be is that the scheduler may be putting too much on one device and not enough on the others if the tasks don't saturate the memory. It'd be nice to avoid that if possible.

[arthurp]

Easier pluggable scheduling might be a good idea. A simple hook that just prioritizes or sorts tasks.