Enhance realtime data filtering for database events

[dstpierre]

The realtime database events always send all data the current user has access to.

For instance, say there's a collection tasks and a user is viewing the completed tasks. The JavaScript client will receive new insert, update, and delete for all tasks records, not just a subset.

It would be easier for developers to specify some filter when subscribing to receive database events.

For the above example, the subscriber could sent completed = true somewhere.

I'm still unclear if this change should be handle in core or directly in the client-side library.

Changing this in core would also impact the server-side function that handles database events, and potentially some non database related events.

This need some exploration. Implementing this in the client-side library would not change the backend. The client-side could handle filtering directly in JavaScript, the down-side is that the websocket connection transmit the data, and it's getting discarded, but might not be a huge issue for < 10k concurren users app.