rgw: fix versioned object IO error

rgw: fix versioned object IO error

    When accessing a copied destination object, its source object's instance ID
    information is needed, however it's missing now in the destination object's
    manifest.

    In order to fix this problem, we can record source object's version_id/instance
    into dest object's manifest(a new filed 'tail_instance' is added). When creating
    a new object(not copy), 'tail_instance' should be equal to its instance value.
    When copy/get a object, 'tail_instance' should always be used to get the right
    tail objects.

    Fixes: http://tracker.ceph.com/issues/17111
    Signed-off-by: Yang Honggang <joseph.yang@xtaotech.com>