Introduce image names and metadata storage in etcd

Each inode has: image name, parent inode number & pool, size and readonly flag

Snapshots are created by switching image name to a different inode number
while using the older inode as parent.

mon/mon.js

@@ -149,6 +149,7 @@ const etcd_tree = {
             <pool_id>: {
                 <inode_t>: {
                     name: string,
+                    size?: uint64_t, // bytes
                     parent_pool?: <pool_id>,
                     parent_id?: <inode_t>,
                     readonly?: boolean,