rgw: translate object marker to raw format

Fixes: #4600
Object marker should be treated as an object, so that name is formatted
correctly when getting the raw oid.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit b083dece36a050ec15ac41a275aeef0ece1ac009)
rgw: escape prefix correctly when listing objects

Fixes: #5362
When listing objects prefix needs to be escaped correctly (the
same as with the marker). Otherwise listing objects with prefix
that starts with underscore doesn't work.
Backport: bobtail, cuttlefish

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