close aio cursors etc. that require await close

close aio cursors etc. that require await close

Improved the base implementation of the asyncio cursor such that it
includes the option for the underlying driver's cursor to be actively
closed in those cases where it requires ``await`` in order to complete the
close sequence, rather than relying on garbage collection to "close" it,
when a plain :class:`.Result` is returned that does not use ``await`` for
any of its methods.  The previous approach of relying on gc was fine for
MySQL and SQLite dialects but has caused problems with the aioodbc
implementation on top of SQL Server.   The new option is enabled
for those dialects which have an "awaitable" ``cursor.close()``, which
includes the aioodbc, aiomysql, and asyncmy dialects (aiosqlite is also
modified for 2.1 only).

Fixes: #12798
Change-Id: Ib17d611201fedf9780dfe3d760760ace99a8835c