Avoid race condition in maintain_leases by copying leased_messages

[theacodes]

No description provided.

[tseaver]

@jonparrott This change is causing unit test failures for me:

_____________________ test_maintain_leases_outdated_items ______________________
Traceback (most recent call last):
  File "/home/tseaver/projects/agendaless/Google/src/google-cloud-python/pubsub/tests/unit/pubsub_v1/subscriber/test_policy_base.py", line 327, in test_maintain_leases_outdated_items
    modify_deadline_seconds=[10],
  File "/home/tseaver/projects/agendaless/Google/src/google-cloud-python/pubsub/.nox/unit-3-6/lib/python3.6/site-packages/mock/mock.py", line 948, in assert_called_once_with
    return self.assert_called_with(*args, **kwargs)
  File "/home/tseaver/projects/agendaless/Google/src/google-cloud-python/pubsub/.nox/unit-3-6/lib/python3.6/site-packages/mock/mock.py", line 937, in assert_called_with
    six.raise_from(AssertionError(_error_message(cause)), cause)
  File "<string>", line 3, in raise_from
AssertionError: Expected call: send_request(modify_deadline_seconds: 10
modify_deadline_ack_ids: "ack2"
)
Actual call: send_request(modify_deadline_seconds: 10
modify_deadline_seconds: 10
modify_deadline_ack_ids: "ack1"
modify_deadline_ack_ids: "ack2"
)
------------------------------ Captured log call -------------------------------
base.py                    382 WARNING  Dropping 1 items because they were leased too long.

[theacodes]

@tseaver if it's blocking you feel free to roll it back until I can take a closer look.,

[tseaver]

I'm not blocked except from helping on pubsub issues.