open up async greenlet for third parties

open up async greenlet for third parties

Modified the internal representation used for adapting asyncio calls to
greenlets to allow for duck-typed compatibility with third party libraries
that implement SQLAlchemy's "greenlet-to-asyncio" pattern directly.
Running code within a greenlet that features the attribute
``__sqlalchemy_greenlet_provider__ = True`` will allow calls to
:func:`sqlalchemy.util.await_only` directly.

Change-Id: I79c67264e1a642b9a80d3b46dc64bdda80acf0aa