rgw: iterate usage entries from correct entry

Fixes: #5152
When iterating through usage entries, and when user id was
provided, we started at the user's first entry and not from
the entry indexed by the request start time.
This commit fixes the issue.

Backport: bobtail

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>