Fix a simple race condition FutureDAGTest.
Fix a simple race condition FutureDAGTest.

Summary:
In the "Complex" test we can have nodes running at the same time. So
appending to the same vector concurrency isn't safe. Protecting that with a
mutex.

Reviewed By: yfeldblum

Differential Revision: D23545746

fbshipit-source-id: b6f6e8b7204cf63c7e8fbbd8fc5134cb66c8180a

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