librbd: allow an image to be opened without watching

librbd: allow an image to be opened without watching

Watching the header of a parent image could produce unreasonable
delays. If hundreds of child images watch the same parent, taking a
snapshot or resizing the parent would wait until all the children are
notified. Since the children are based on snapshots, they don't care
about any changes to the current version of the parent image, and
don't need to re-read the header on each change. Nothing children need
to access their parent snapshot will change.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>