librbd: complete all pending aio ops prior to closing image

It was possible for an image to be closed while aio operations
were still outstanding.  Now all aio operations are tracked and
completed before the image is closed.

Fixes: #10299
Backport: giant, firefly, dumpling
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
we can assert this instead, it should only ever be called once