folly::Future integration with Python
folly::Future integration with Python

Summary:
This diff implements two functions which can be used to bridge C++ asynchronous APIs with Python.
1. getExecutor(event_loop) allows extracting a folly::Executor-compatible object from Python event_loop.
2. wrapFuture wraps any folly::Future object in a Python Future object.

Reviewed By: ambv

Differential Revision: D2878731

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