Handle recursive objects or throw an exception.

[robertnishihara]

The following leads to an infinite loop (and hence a segfault).

import ray
ray.init(start_ray_local=True, num_workers=0)

l = []
l.append(l)
ray.put(l)  # This causes a segmentation fault.

The way we serialize objects currently does not allow for recursive objects.

[robertnishihara]

This is addressed by #440.