rgw: consider this scenario, many sibling directories, and many objects under each...

rgw: consider this scenario, many sibling directories, and many objects under each directory.
if cls_filtered is true, tracker's entries may be the same as each other
the next_candidate operation may advance to end directly,
and only one object may be returned, this will slow down when listing sibling directories.

Signed-off-by: zealot <xzd16@sohu.com>
(cherry picked from commit 678dc395dd82ca77e46aa665a4a0c7d918c538f3)

Conflicts:
	src/rgw/rgw_rados.cc

Cherry-pick notes:
- "stopped accumulating results" log line wasn't present in Pacific