rgw: add streamlined ways to handle indexless buckets correctly

rgw: add streamlined ways to handle indexless buckets correctly

Determining whether a bucket is indexless starting with an
RGWBucketInfo object requires traversing multiple data structures and
"inside knowledge" blurring the line between interface and
implementation. The same applies for retrieving the current index for
non-indexless buckets.

This commit adds to the RGWBucketInfo interface to make this
information readily accessible.

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