utf8: export encode_utf8() and decode_utf8()

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: improve delmited listing of bucket

Still not complete, as string calculation need to consider utf8
encoding.

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>