We sometimes to need to call another method in asyncio context when we already are in this
context. With the `@aio` decorator which makes the method return a `Deferred`, it was
necessary to use a ping pong between `Deferred` and `Future`.
This patch adds a `aio_func` attribute which directly returns the wrapped asyncio method,
so it can now be awaited directly.
`maybe_async` can be used to call a function which may be or not `async`. It doesn't use
`defer.ensureDeferred` contrary to `ad_defered`, so the call stack is not lost, and it's
easier to debug when an Exception is raised.
