refactor: add networkengine framework.

[DrmagicE]

This PR mainly makes the following changes:

1. Decouple route and VPN implementation.
2. Rename vxlan interface name raven to raven0.
3. Calculate the desired network state from Gateway list on each change. This prevents mismatched state when the delete event is missed during watch connection getting reestablished.
4. Enable loose mode reverse path filtering on raven0.
5. Configure reverse-path route to avoid asymmetric routing.
6. Add src field in route entries to preserved node cni IP.

fix #30

[njucjc]

It seems that the configuration for fdb is missing:

• on gateway node 00:00:00:00:00:00 dev raven0 dst <non_gateway_private_ip> self permanent
• on non-gateway node 00:00:00:00:00:00 dev raven0 dst <gateway_private_ip> self permanent

Without this configuration, the cross-edge traffic of the non-gateway node cannot be forwarded to the gateway node.

reference: http://just4coding.com/2020/04/20/vxlan-fdb/

[DrmagicE]

It seems that the configuration for fdb is missing:

• on gateway node 00:00:00:00:00:00 dev raven0 dst <non_gateway_private_ip> self permanent
• on non-gateway node 00:00:00:00:00:00 dev raven0 dst <gateway_private_ip> self permanent

Without this configuration, the cross-edge traffic of the non-gateway node cannot be forwarded to the gateway node.

reference: http://just4coding.com/2020/04/20/vxlan-fdb/

I miss that, will fix it today.

[DrmagicE]

It seems that the configuration for fdb is missing:

• on gateway node 00:00:00:00:00:00 dev raven0 dst <non_gateway_private_ip> self permanent
• on non-gateway node 00:00:00:00:00:00 dev raven0 dst <gateway_private_ip> self permanent

Without this configuration, the cross-edge traffic of the non-gateway node cannot be forwarded to the gateway node.

reference: http://just4coding.com/2020/04/20/vxlan-fdb/

Updated and tested, please have a look. https://github.com/openyurtio/raven/compare/a2c3d49c06294a11081b28e43b5839848f4d1e51..e46cca7c4a3b2afd547221be45e201a08654b003

[njucjc]

LGTM