fix AsyncTests.*_Close (await pending futures when closing the session)

[muzarski]

As the issue title suggests - cpp-driver (most probably - need to investigate how it's done!!!) awaits all pending futures when closing the session (cass_session_close).

The test mentioned in the title strongly suggests such behavior on cpp-driver side. We should do it as well.

[muzarski]

@dkropachev @wprzytula I'll let you decide on the priority (and milestone) for this item. For me, it looks like P2. It would be nice to have as a part of 1.0.0. While it seems like a niche use case (without it, user can still await futures with cass_future_wait after closing the session), there may be users who depend on this behavior.