rgw: don't iterate through all objects when in namespace

Fixes: #4363
Backport: argonaut, bobtail
When listing objects in namespace don't iterate through all the
objects, only go though the ones that starts with the namespace
prefix

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 6669e73fa50e3908ec825ee030c31a6dbede6ac0)