[refactor] split routeWatcher into watcher and handler components

[aritrbas]

This refactors the RouteWatcher component by separating the route watching functionality from the route handling logic.

Previously, the RouteWatcher used to (i) monitor netlink route updates and address changes and (ii) respond to network and IPAM configuration events via pubsub. Because of this, the RouteWatcher needed to be passed to the NetWatcher just so it could subscribe to events, and the event handling logic was mixed with route watching logic.

Now, we have a new handler component RouteHandler that implements three event handling methods:

• OnNetDeleted(netDef *common.NetworkDefinition) error
• OnNetAddedOrUpdated(netDef *common.NetworkDefinition) error
• OnIpamConfChanged(oldPool, newPool *proto.IPAMPool) error

and holds a reference to RouteWatcher to perform route operations.

The RouteWatcher is now purely focused on route watching. It no longer subscribes to pubsub or performs any event handling. It is only responsible for netlink route/address monitoring.

NetWatcher and IPAM configuration changes now directly call the RouteHandler instead of sending pubsub events resulting in network and IPAM events being synchronously processed when they occur.

NetWatcher no longer needs to know about RouteWatcher, it only knows about the RouteHandler interface.

Events are now processed directly: NetWatcher → RouteHandler → RouteWatcher
Previously: NetWatcher → SendEvent → PubSub → RouteWatcher.eventChan → RouteWatcher

Made the changes on top of the nsk-split-svc branch for now, will rebase on top of master once those commits for single thread agent are merged.

[sknat]

thanks ! A few comments inline,
I think we could split the uplink_route_watcher in half and simplify the logic even more,
tell me if things are unclear !