radosgw-admin: wait before trimming usage

Fixes: #9892

Need to wait through the usage interval before trimming usage, otherwise we might not
remove all pending usage info.

Backport: dumpling, firefly, giant

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
(cherry picked from commit dd09ecbfab8a659f3faaf879a52849caab5e8e8e)
Merge pull request #266 from ceph/wip-9892-2

radosgw-admin: wait before trimming usage