batch/debounce websocket set_props

[T4rk1n]

When you use set_props in a loop inside a websocket callback, it would create single updates in the renderer and can result in lag.
This PR add a batching and debounce mechanism to set_props, default to 5ms/200hz configurable with

Dash(..., websocket_batch_delay=0.005)  # 5ms (default) - set to 0 to disable batching