mwan3: mwan3_set_connected not updated when new routes are added

[aaronjg]

Maintainer: @feckert
Environment: 19.07.3, mwan3 2.9.0 and before

Description:
mwan3 has special handling for addresses that have explicit routes in the main routing table. For these routes, the default route is used, and when multiple routes are available, the interface is chosen by the kernel based on the metric in the main routing table.

It handles this by using ipsets called mwan3_connected_v{4|6}. However, this is never updated when new routes are added. (though it is updated when new interfaces come online). This can cause a problem for tunneling interfaces that set up a direct route to the host before coming online using proto_add_host_dependency

@feckert - feel free to assign to me. Working on a fix for this now.

[aaronjg]

With a faster startup, this seems more likely to occur than before, so I added a fix to: #13169 by having rtmon update the ipsets when new routes are added/deleted.