rgw: prevent spurious/lost notifications in the index completion thread

rgw: prevent spurious/lost notifications in the index completion thread

this was happening when asyn completions happened during reshard.
more information about testing:
https://gist.github.com/yuvalif/d526c0a3a4c5b245b9e951a6c5a10517

we also add more logs to the completion manager.
should allow finding unhandled completions due to reshards.

Fixes: https://tracker.ceph.com/issues/54435
Signed-off-by: Yuval Lifshitz <ylifshit@redhat.com>
(cherry picked from commit e72b6510a402cbafa6c99475626f15d97fd00f86)