rgw: add NextMarker param for bucket listing

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>