Introduce a session for a device connection & stats

[teon]

Related to #1699

[wojcik91]

• add a dedicated component for managing vpn client sessions
• pass all stats messages from gateways to this component
• pre-process and store stats in a new DB table
  • create sessions based on incoming stats
  • all stats should be attached to an existing session
• add tests for stats
• use new sessions table to list connected devices in location overview
• use new stats table to show connection stats in location overview
• handle disconnecting inactive MFA sessions in connection manager
• move vpn client connect/disconnect events to connection manager
• add tests for tracking VPN clients connections
• disconnect all sessions on device modification