rgw: clean up interenal shard logic in ordered bucket list

rgw: clean up interenal shard logic in ordered bucket list

The logic of RGWRados::cls_bucket_list_ordered is complex as it needs
to query multiple bucket index shards and manage their returned lists,
and walk through them in a manner to produce an appropriately ordered
set of dir entries to the caller. The previous implementation used
three of parallel std::vectors (e.g., the related data was spread
across the vectors), which needed to each be maintained
appropriately. This clean-up changes this to a single vector of
structs, and the struct both consolidates the related data and
encapsulates much of the related logic.

Additionally, const correctness is expanded.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>