Notify main app about restored sessions (aka Restore RPC)

[palkan]

Related to the resumable sessions feature (v1.4).

Right now, if session has been restored, we do not notify a Ruby app about it. So, the lifecycle looks as follows:

connect      -> RPC Connect
disconnect   -> RPC Disconnect
restored     -> ??
disconnect   -> RPC Disconnect

So, we're not only missing the "connect" (or "restore") event, but also we may have two "disconnect" events.

It seems that we need to add a new RPC method (Restore?) and also trigger connection/channel class callbacks (reconnect and resubscribed?).

What are the other options? Postpone Disconnect calls till the session cache expired—not good, the TTL can be minutes—too long.