Feature #11766: rbd-mirror: basic structure
RBD mirroring settings should be moved to new pool object
The current 'rbd_pool_settings' object which holds whether or not mirroring is enabled for the pool (and its peers) should be renamed to 'rbd_mirroring' and should be expanded to include a directory if which images are mirrored within the pool and a state mapping to remote peers (remote ID, whether a deletion has propagated, etc).
cls_rbd: add methods for keeping track of mirrored images
These will track whether local images should be mirrored, and map them
to a unique global id. There's a state field for safely disabling
mirroring while operating on multiple objects.