Handle MFA session connect/disconnect in the session manager

[wojcik91]

• create session with state 'new' once MFA process is completed
• update session to connected once stats are received
• disconnect inactive MFA sessions once they become inactive
• remove new sessions if no stats are received
• emit dedicated MFA client connected/disconnected events in the session manager
• remove existing periodic disconnect task

Results:

• run_periodic_peer_disconnect loop is removed
• clients can connect to the dev instance with MFA
• once disconnected the session is marked as disconnected and peer is removed from the gateway
• MFA connect/disconnect events can be seen in the audit log
• there is no duplicate VPN client connected/disconnected event in the audit log