librbd: collapse API mirror primary/non-primary namespaces to single type

librbd: collapse API mirror primary/non-primary namespaces to single type

Eventually we will want to be able to chain multiple peers together.
In order to avoid special logic for syncing non-primary snapshots from
a peer, just treat all mirror snapshots the same.

This also clears some confusion for how to handle demoted snapshots
since otherwise it would involve creating a primary-snapshot on the
non-primary image before the image was fully synced.

Finally, the copied flag was renamed to completed since we should
eventually only set this to true on primary snapshots after the
image state has been created.

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