Rename client status to data, add group data.

We now distinguish between status, which is maintained by the server,
and data, which is provided by the client.  In addition to client data,
we now support group data.

diskwriter/diskwriter.go

@@ -69,11 +69,11 @@ func (client *Client) Permissions() group.ClientPermissions {
 	}
 }
 
-func (client *Client) Status() map[string]interface{} {
+func (client *Client) Data() map[string]interface{} {
 	return nil
 }
 
-func (client *Client) PushClient(group, kind, id, username string, permissions group.ClientPermissions, status map[string]interface{}) error {
+func (client *Client) PushClient(group, kind, id, username string, permissions group.ClientPermissions, data map[string]interface{}) error {
 	return nil
 }