Description of problem: Currently rbd-mirror image status doesn't depict how many snapshots are in queue. In our observations, some images were behind 3 snapshot to be synced. RBD image status doesn't reflect the same. It would be nice to display information on current different between last snapshot 100% copied snapshot and current time. Or image status could depict that the mirror image snapshot queue has more than 1 image. This would be helpful for users to compare it with their RPO. Version-Release number of selected component (if applicable): <latest> Steps to Reproduce: 1. Configure snapshot based mirroring b/w two clusters. 2. Deploy agressive IOs on images or try to have controlled lesser IOs b/w two clusters. 3. check rbd snap ls --all on images in primary - there would be queue of 3-5 snaps for some of the images check snap ls --all on secondary, there would be two snapshots, one 100% copied, and one in progress which could be copy of one of a snapshot in queue of primary. 4. check rbd image status at both primay and secondary. Image status doesn't information on time since last 100% copied snapshot. Actual results: and Expected results: (Explained above)