Attaching another patch. Please review (I plan to commit it tomorrow in 3.5 and 3.6 branches).
The patch affects generators machinery in the following way:
1. Generators behaviour isn't touched, the patch is only for 'async def' coroutines.
2. Calling 'send()' or 'throw()' on a coroutine object after it is exhausted or closed triggers a `RuntimeError("coroutine was already awaited")`
3. Calling 'close()' method on an exhausted or closed coroutines is a no-op. 'close()' can be called multiple times -- same as for generators.