Fix Plasma starting failure when specify the memory in float value.

[guoyuhong]

What do these changes do?

When we set a float value to object_store_memory to do ray.init(object_store_memory=XXX), Plasma store won't start. We need to guarantee that object_store_memory is an integer.

Related issue number

2333

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[robertnishihara]

Instead of modifying worker.py, I think it makes more sense to raise an exception in

ray/python/ray/plasma/plasma.py

Line 24 in bb1d7ea

def start_plasma_store(plasma_store_memory=DEFAULT_PLASMA_STORE_MEMORY,

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[guoyuhong]

Thanks, @robertnishihara .