- BFD model should have separate interval setting for RX and TX, hence
`bfd_interval` -> `bfd_interval_rx`, `bfd_interval_tx`;
- BFD model is only relevant to the SBP interface (with all attributes);
- BGP session will only have a simple flag `bfd_enabled` with default
`False`;
- BFD class models moved from `bgp_session.py` to
`service_binding_port.py`.