Make SingleValue.update thread-safe
Make SingleValue.update thread-safe

Use a @@class_var (shared with subclasses) rather than an @instance_var (unique to each class)
even though there will be no race condition between classes/types, so each subclass does
not need to initialize @mutex.

nilbus/pinewood-derby,nilbus/pinewood-derby,nilbus/pinewood-derby,nilbus/pinewood-derby