rgw: add NextMarker param for bucket listing

Partially fixes #8858.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: account common prefixes for MaxKeys in bucket listing

To be more in line with the S3 api. Beforehand we didn't account the
common prefixes towards the MaxKeys (a single common prefix counts as a
single key). Also need to adjust the marker now if it is pointing at a
common prefix.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: list extra objects to set truncation flag correctly

Otherwise we end up returning wrong truncated value, and no data on the
next iteration.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: dump prefix unconditionally

As part of issue #8858, and to be more in line with S3, dump the Prefix
field when listing bucket even if bucket is empty.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>