tiered storage: can copy to s3, read from s3

master not aware tiered volume yet, file assigning is not working yet

weed/pb/volume_server_pb/volume_server_helper.go

@@ -0,0 +1,5 @@
+package volume_server_pb
+
+func (m *RemoteFile) BackendName() string {
+	return m.BackendType + "." + m.BackendId
+}