librbd: differentiate between R/O vs R/W RBD features

The new RBD exclusive lock feature should be treated as a
feature that is only applied when the image is opened in
R/W mode.

Older clients will need to handle the updated
cls_rbd::get_features method in order to properly determine
the incompatible features for an image depending on the
current mode.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>