Emit VPN client connected/disconnected audit log events from the session manager

Currently those events are still generated in the GatewayHandler and are based on an in-memory client map. Reimplement this logic in the session handler based on the new DB tables.

Result: GatewayHandler no longer includes any code related to tracking connected clients and emitting events. Audit log events for VPN client connections can still be observed in the log as before.