refactor: move various duties from ChannelRouter to graph.Builder

This commit is a large refactor that moves over various responsibilities
from the ChannelRouter to the graph.Builder. These include all graph
related tasks such as:
- graph pruning
- validation of new network updates & persisting new updates
- notifying topology update clients of any changes.

This is a large commit but:
- many of the files are purely moved from `routing` to `graph`
- the business logic put in the graph Builder is copied exactly as is
  from the ChannelRouter with one exception:
- The ChannelRouter just needs to be able to call the Builder's
  `ApplyChannelUpdate` method. So this is now exported and provided to
the ChannelRouter as a config option.
- The trickiest part was just moving over the test code since quite a
  bit had to be duplicated.

routing/payment_lifecycle.go

@@ -912,7 +912,7 @@ func (p *paymentLifecycle) handleFailureMessage(rt *route.Route,
 	}
 
 	// Apply channel update to the channel edge policy in our db.
-	if !p.router.applyChannelUpdate(update) {
+	if !p.router.cfg.ApplyChannelUpdate(update) {
 		log.Debugf("Invalid channel update received: node=%v",
 			errVertex)
 	}