common/LogClient: fix sending dup log items

We need to skip even the most recently sent item in order to get to the
ones we haven't sent yet.

Fixes: #9080
Backport: firefly, dumpling
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2243 from ceph/wip-log-client

common/LogClient: fix sending dup log items

Reviewed-by: Loic Dachary <loic@dachary.org>