DittoSignalNextCallback

The signal interface passed to Java callbacks by observeLocalWithNextSignal calls. Calling signalNext tells Ditto the observer's handler is ready to receive another callback. This is a mechanism for handling backpressure when updates arrive faster than the handler can process them. Ditto pauses further callbacks, but continues to apply changes to the local store. If multiple relevant changes occur while paused, Ditto does not queue a callback for each change. After signalNext is called, Ditto calls the handler once with the latest result if that result differs from the last result delivered.