pybind/rados: track completions before calling aio functions

pybind/rados: track completions before calling aio functions

Tracking completions is critical for memory safety - if the
aio function succeeds, the completion must be tracked. However,
if a KeyboardInterrupt or similar arrives between the call and
the tracking, the completion will not be tracked.

Fix this by tracking the completion before the aio call, and
explicitly cleaning up in the failure case.

This leaves the opposite problem, where an unexpected exception
(other than simple error return from the aio function) will cause
the completion to not be freed until the Ioctx is destroyed, but
that is a relatively minor issue.

Signed-off-by: Hector Martin <marcan@marcan.st>