Convert to async/await syntax

[balloob]

We're on Python 3.5 so it's time to let go of @asyncio.coroutine. Migration of a piece of code is simple as they are functionally compatible:

• Remove @asyncio.coroutine decorator
• Add async before function definition: async def async_start(self):
• Replace all yield from with await

For now, let's focus on updating all pieces of code for which we have tests. Let's not update the tests and the code in the same PR. Instead, do 1 PR for the code, 1 PR for the tests. That way we know we don't break things.

If you see an error that you can't await a generator context, it's because you're awaiting a function that doesn't have @asyncio.coroutine as the first decorator above the function name. In such cases, just replace decorator with async and you're good 👍

Python 3.5 also does a better job in warning when async methods have been called but not awaited. When you run py.test locally, note these warnings and please fix them 👍

To kick it off, here is a conversion of the core: #12610

[pvizeli]

Also all asyncio.Locks change the syntax while they support async context manager syntax.

[balloobbot]

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