rgw: improve delmited listing of bucket

If found a prefix, calculate a string greater than that so that next
request we can skip to that. This is still not the most efficient way to
do it. It'll be better to push it down to the objclass, but that'll
require a much bigger change.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit e6cf618c257f26f97f60a4c1df1d23a14496cab0)
rgw: add NextMarker param for bucket listing

Partially fixes #8858.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 924686f0b6593deffcd1d4e80ab06b1e7af00dcb)
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>
(cherry picked from commit 82d2d612e700f94a0bb2d9fb7555abf327be379b)
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>
(cherry picked from commit dc417e477d4ad262885c6b5f5987cf06d63b159d)
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>
(cherry picked from commit d7209c11251d42227608bc54cc69232ef62ffe80)

Conflicts:
	src/rgw/rgw_rest_s3.cc