- Update the flag endpoint->failover_reg_addr with 3 states, 0/1/2 ->
  none/504-register-ongoing/504-registerfailed
- Add the 504_flag into the pjsip show registrations which will be used by
  chan-voicemngr to decide if an INVITE should be sent after 504.
- Restore the original client_state->registration_expires if 504-register failed.