Provide experimental API for changing number of return values and res…

[robertnishihara]

…ource requirements at task submission time.

@devin-petersohn can you try this out and see if it does what you want? You can use it with

import ray

ray.init()

@ray.remote
def f(n):
    return n * [1]

a, b, c = f._submit(args=[3], num_return_vals=3)

TODO before merging:

• Add tests.
• Remove code duplication between ._submit and .remote.
• Make sure this doesn't mess up the error messages.
• Make sure it doesn't affect performance.

[AmplabJenkins]

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