Summary:
This implements a new AsyncioLoopController for FiberManager, wrapping AsyncioExecutor.
Thanks to recent simplication of LoopController, if I haven't missed anything, code is fairly simple, though not optimizer for thread-local.

fiber_manager.pxd|pyx allow to link the life-time of AsyncioLoopController/FiberManager to the asyncio event loop.
Code is similar to (and rely on) executor.pxd|pyd, which does the same for AsyncioExecutor.

fibers.h|pxd provide a helper function for C++/python callers.
Code is similar to futures.h|pyd, which does the same for folly Future.
This is the main reason to keep a separate function/callback parameters. Not necessary for fibers, but a generic callback implementation can work for both fibers/future, which is convenient.

There are a few lines that feel duplicated between future/fibers implementation, but it felt overkill to abstract further.

Reviewed By: andriigrynenko

Differential Revision: D7811432

fbshipit-source-id: af0a07d7554acbab1fac44b7a9a6a98340501ef4