Fix bug in ray.wait

[stephanie-wang]

What do these changes do?

ray.wait depends on callbacks from the GCS to decide when an object has appeared in the cluster. The raylet crashes if a callback is received for a wait request that has already completed, but this actually can happen, depending on the order of calls. More precisely:

1. Objects A and B are put in the cluster.
2. Client calls ray.wait([A, B], num_returns=1).
3. Client subscribes to locations for A and B. Locations are cached for both, so callbacks are posted for each.
4. Callback for A fires. The wait completes and the request is removed.
5. Callback for B fires. The wait request no longer exists and raylet crashes.

[AmplabJenkins]

Test FAILed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/Ray-PRB/9683/
Test FAILed.

[robertnishihara]

@stephanie-wang is there a good way to test this?