rados.py: keep reference to python callbacks

rados.py: keep reference to python callbacks

If we don't keep a reference to these, the librados aio calls will
segfault since the python-level callbacks will have been garbage
collected. Passing them to aio_create_completion() does not take a
reference to them. Keep a reference in the python Completion object
associated with the request, since they need the same lifetime.

This fixes a regression from 60b019f69aa0e39d276c669698c92fc890599f50.

Fixes: #10775
Backport: dumpling, firefly, giant
Signed-off-by: Josh Durgin <jdurgin@redhat.com>
(cherry picked from commit 36d37aadbbbece28d70e827511f1a473d851463d)