Fix detecting folder hierarchy changes in FolderSync (Bifrost#T42645)

It happened that hasHierarchyChanges() detected changes properly, but
when a device sent FolderSync to get the changes we returned empty
result. It's because we didn't (and we really couldn't) implement
getChangedFolders(). Now we compare client and server folders'
properties to make sure all changes are returned back to the device.