use ensure_closed() for async close, close() for terminate

use ensure_closed() for async close, close() for terminate

Fixed issue in asyncio dialects asyncmy and aiomysql, where their
``.close()`` method is apparently not a graceful close.  replace with
non-standard ``.ensure_closed()`` method that's awaitable and move
``.close()`` to the so-called "terminate" case.

Fixes: #10893
Change-Id: I33d871e67854d85f770c46f699e41a6e73b6fbe0