Add support for subscriptions

[rolandjitsu]

Currently, all events broadcasted from the server are sent over the wire to all clients. While this is expected behaviour, it is sometimes undesirable.

I propose using the pub/sub pattern where clients can subscribe to specific RPC methods and only receive messages for those methods that the client subscribed to.

By default, the server, would send messages for a specific method only to the clients that subscribed to that method.

E.g. of subscribe RPC:

{
    "jsonrpc": "2.0",
    "method": "subscribe", // "unsubscribe" when user don't want messages for the method anymore
    "params": {
        "method": "SOME_METHOD"
    }
}

[tolokoban]

I agree! That's exactly what I need too.