graceful mid_process() timeout handling

[mmoskal]

As per discussion:

When mid_process() exceeds deadline, pretend mid_process() returned backtrack=0,tokens=[],no_sampling (so no-op) and wait for it to actually return (some number of steps - this gets into economics of how a cloud provider charges for running the controller). Of course, we would want to avoid too many sequences in a batch stalling that way