ensure greenlet_spawn propagates BaseException

ensure greenlet_spawn propagates BaseException

Fixed bug in asyncio implementation where the greenlet adaptation system
failed to propagate ``BaseException`` subclasses, most notably including
``asyncio.CancelledError``, to the exception handling logic used by the
engine to invalidate and clean up the connection, thus preventing
connections from being correctly disposed when a task was cancelled.

Fixes: #6652
Change-Id: Id3809e6c9e7bced46a7a3b5a0d1906c4168dc4fc