rgw: reduce per-shard entry count during ordered bucket listing

rgw: reduce per-shard entry count during ordered bucket listing

Currently, if a client requests the 1000 next entries from a bucket,
each bucket index shard will receive a request for the 1000 next
entries. When there are hundreds, thousands, or tens of thousands of
bucket index shards, this results in a huge amplification of the
request, even though only 1000 entries will be returned.

These changes reduce the per-bucket index shard requests. These also
allow re-requests in edge cases where all of one shard's returned
entries are consumed. Finally these changes improve the determination
of whether the resulting list is truncated.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>
(cherry picked from commit 729c0ba1361b76ecace94793b956354486b3dcd8)