rgw/rados: fix init/shutdown order for RGWIndexCompletionManager

rgw/rados: fix init/shutdown order for RGWIndexCompletionManager

RGWRados::init_complete() initializes this RGWIndexCompletionManager
after starting some background threads that depend on it, like RGWLC and
RGWDataSyncProcessorThread and RGWObjectExpirer. this can lead to a
crash when accessing a null index_completion_manager

RGWRados::finalize() deletes it before stopping the Restore thread

Fixes: https://tracker.ceph.com/issues/74067
Reported-by: J. Eric Ivancich <ivancich@redhat.com>
Signed-off-by: Casey Bodley <cbodley@redhat.com>