Fix rlib test failure caused by deadlock

[guoyuhong]

What do these changes do?

Add lock in fetch_and_execute_function_to_run of import_thread.py.
import_thread.py and main thread may both call pickle.loads to import a certain class. If they do the importing at the same time, there will be exception. Thus, the lock is necessary.

2019-04-29 06:12:41,440	ERROR worker.py:1672 -- Failed to unpickle actor class 'PolicyEvaluator' for actor ID 50e34046a1fe558cbeaf18b48516ab015b832bc6. Traceback:
Traceback (most recent call last):
  File "/home/travis/.local/lib/python3.6/site-packages/ray-0.7.0.dev3-py3.6-linux-x86_64.egg/ray/function_manager.py", line 729, in _load_actor_class_from_gcs
    actor_class = pickle.loads(pickled_class)
  File "/home/travis/.local/lib/python3.6/site-packages/ray-0.7.0.dev3-py3.6-linux-x86_64.egg/ray/rllib/__init__.py", line 11, in <module>
    from ray.rllib.evaluation.policy_graph import PolicyGraph
  File "/home/travis/.local/lib/python3.6/site-packages/ray-0.7.0.dev3-py3.6-linux-x86_64.egg/ray/rllib/evaluation/__init__.py", line 2, in <module>
    from ray.rllib.evaluation.policy_evaluator import PolicyEvaluator
  File "/home/travis/.local/lib/python3.6/site-packages/ray-0.7.0.dev3-py3.6-linux-x86_64.egg/ray/rllib/evaluation/policy_evaluator.py", line 21, in <module>
    from ray.rllib.evaluation.sampler import AsyncSampler, SyncSampler
ImportError: cannot import name 'AsyncSampler'
2019-04-29 06:12:41,440	ERROR worker.py:1672 -- Traceback (most recent call last):
  File "/home/travis/.local/lib/python3.6/site-packages/ray-0.7.0.dev3-py3.6-linux-x86_64.egg/ray/import_thread.py", line 128, in fetch_and_execute_function_to_run
    function = pickle.loads(serialized_function)
  File "/home/travis/.local/lib/python3.6/site-packages/ray-0.7.0.dev3-py3.6-linux-x86_64.egg/ray/rllib/evaluation/sampler.py", line 15, in <module>
    from ray.rllib.evaluation.tf_policy_graph import TFPolicyGraph
  File "/home/travis/.local/lib/python3.6/site-packages/ray-0.7.0.dev3-py3.6-linux-x86_64.egg/ray/rllib/evaluation/tf_policy_graph.py", line 14, in <module>
    from ray.rllib.evaluation.policy_graph import PolicyGraph
  File "/home/travis/.local/lib/python3.6/site-packages/ray-0.7.0.dev3-py3.6-linux-x86_64.egg/ray/rllib/__init__.py", line 11, in <module>
    from ray.rllib.evaluation.policy_graph import PolicyGraph
  File "/home/travis/.local/lib/python3.6/site-packages/ray-0.7.0.dev3-py3.6-linux-x86_64.egg/ray/rllib/evaluation/__init__.py", line 2, in <module>
    from ray.rllib.evaluation.policy_evaluator import PolicyEvaluator
  File "/home/travis/.local/lib/python3.6/site-packages/ray-0.7.0.dev3-py3.6-linux-x86_64.egg/ray/rllib/evaluation/policy_evaluator.py", line 21, in <module>
    from ray.rllib.evaluation.sampler import AsyncSampler, SyncSampler
ImportError: cannot import name 'AsyncSampler'

Related issue number

https://travis-ci.com/ray-project/ray/jobs/196114212
#4499

Linter

• I've run scripts/format.sh to lint the changes in this PR.

[AmplabJenkins]

Test PASSed.
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Test PASSed.

[AmplabJenkins]

Test FAILed.
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Test FAILed.