Fix bug in import thread.

[robertnishihara]

In some cases, functions exported from the driver are run on the driver twice (once right when they are exported and then once when they are later imported on the same driver). We should not be importing them at all and definitely should not be executing them a second time.

Before this PR:

import ray
import sys

ray.init()

def f(x):
    sys.path.append(1)

ray.worker.global_worker.run_function_on_all_workers(f)

import time
time.sleep(1)

print(sys.path)  # This used to end with [1, 1] before this PR. It should be [1].

Related to #2406.

[robertnishihara]

cc @surehb

[AmplabJenkins]

Test FAILed.
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Test FAILed.