Before this commit, V2 listing parameters ('start-after',
'continuation-token' and 'fetch-owner') were just ignored, making some
S3 clients return errors, or handle paging badly.
V2 listing is selected by passing 'list-type=2' in query string.
'marker' is replaced by either 'start-after' or 'continuation-token'.
This commit wraps 'start-after' and 'continuation-token' in 'marker',
which is passed to swift. 'NextContinuationToken' is a base64 encoding
of the last returned object, so it is opaque to the client.
Change-Id: I23bf83cb8bbaf4c4935bf6b56791051c032c688c
69313a69bf