cls_ver: rename version xattr, add some more logging

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: add str_startswith()

useful util

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: filter read xattrs

We're only interested in object xattrs that have specific rgw.user
prefix.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: fixes to object versioning tracking

There are a few different cases for setting the object version.
Either we need to create a new version, or we need to set the
version provided (one metadata put). We also need to make sure
that we log the correct previous version of the object. This
commit fixes a few cases.

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