cache data server to serve updates asynchronously for better responsiveness

[webbpinner]

** Disclaimer - I do not claim to fully understand the inner workings of OpenRVDAS, what I'm requesting may already implemented or completely impossible to implement

The current cached data server serves websocket-connected clients with updated data from the data streams that the clients have subscribed to. The updated are provided synchronously at a default rate of 1Hz. The updates arrive regardless if the underlying data has changed since the previous update was sent.

This creates two scenarios that I feel should be avoided if possible:

1. Connected clients receive "new" data even if the data hasn't changed.
2. The minimum refresh interval of the client is controlled by the broadcast interval of the cache data server and not by changes in the underlying data.

Recommend refactoring the cache-data-server to send connected clients with updated data when the underlying data changes vs at a server-defined update rate.